Pros:
Seneca is a spectacular course, and unlike any other I know of. This is the only place I know of where the 2-meter rule is still in effect, and will always be in effect. And before people start grousing that the 2-meter rule is an unnecessary or unfair penalty, let me explain a little about Seneca:
Hole design at Seneca is dependent on an amazing number of really beautiful and devilish cedar trees. About half the holes are strategically designed so that cedar trees become a serious obstacle and a penalty to shots that miss the fairway or miss the right line to the basket. And these are sticky, sticky cedars. I'd estimate that 1/3 of discs that hit cedars stick in the cedars. So there is a sharp incentive to avoid them. And the risk/reward of having the 2-meter rule in effect on these cedars adds immensely to the challenge of the course.
To offset the effect of the cedars, Seneca is the best rolling course I have ever played. The holes with cedars almost invariably have amazingly well-groomed fairways with well-trimmed grass and very, very few roots or other debris on the ground. Rollers are glorious here, and the rolling hills throughout the course meant that rollers have to be carefully placed -- you can't just huck a roller out and hope for the best -- you have to learn finesse. And I know of no other course in the country that will be so helpful in teaching you how rollers should work and what discs can do when they hit the ground.
The half of the course that isn't cedars/rollers is mostly beautiful shots along a shallow creek with 150' trees towering overhead. Wildlife is plentiful -- I've seen countless deer, as well as foxes, beavers, racoons, turtles, snakes, lizards, etc... around the course -- most of them near the water. These shots don't have cedars to worry about, but the OB creek forces precision.
Other reviewers have mentioned that the 27-hole layout favors lefty throwers, and I have to disagree with that. Seneca does require a good balance of right-and-left turning holes, but the holes that are "lefty holes" almost always have very high-ceilings and wide enough fairways that there is no trouble throwing an anhyzer shot. To me, the only time a course becomes a "lefty" course is when there are a lot of low-ceiling, right-hooking holes that mandate a lefty-shot to have a birdie-run. Here, that's not the case. Holes are fair to every player and usually offer plenty of room to get to the hole by several options. And there are a lot of very hard-3s (or easy-4s), where shot placement for the 3 is much more important than trying to hit a deuce. From the long pins, you can shoot ~970 golf here without every taking a deuce. Which is just awesome.
Other pros for the course: Three nine-hole loops, multiple tees and pin-placements for every hole, incredibly-well maintained, good parking, dedicated disc-golf-only area, so there are never any hikers or other folks competing for space, and perhaps foremost: a great bunch of locals dedicated to the constant improvement of the course.
Seneca is the closest A-list course to my house (about 45-50 minutes without traffic), and in the two-years I've lived here, I've seen it go from an okay set of 18-holes to a Pro-level 27-hole course that improves every time I make it out.
While there are still some holdouts who favor elements of the old layout, the redesigned course improves many elements of the course and has also served to provide a stronger draw for players of all skill levels.
The volunteers who maintain and continue to improve the park (can't name them all here -- I don't want to leave people out) have done an amazing job, over the last year especially, continuing to improve what was already an epic course. New signage (AMAZING) and a few more pin placements secure the course a 4.5-star rating, placing it among my favorite places to play in the country.
Each hole has an average of 3 teeboxes (though the yellow novice pads are VERY short and not always well marked (though that is getting better), and usually 2-4 basket placements, which means there is a lot of variation to the course. The recent addition of screws/hooks to indicate current pin position is a huge help.
The park itself is incredibly well maintained and has lots of other activities as well -- a perfect place to spend a day with a family or plan a family reunion, company picnic or other large get together.
Cons:
It's hard to find many cons for Seneca. The biggest one for me is that there are not multiple baskets on every hole. That could take this course to a 5-star rating. The course has so many possibilities that it just begs for a permanent "longs" layout (similar to Patapsco, also in MD). The long-to-long layout on this course is truly awesome and a tremendous challenge. But with only one pin per hole, it is never set up in it's most challenging layout except for occasional tournament rounds. This is in keeping with the goal of appealing to varying skill levels of players, which is a great goal. But in a perfect world, this course would have permanent long pins.
Other minor issues are inconsistent signage. The new signs are among the best I've ever seen at a course (check out the pictures), but they are not on every teebox, and are not consistently on a certain teebox (sometimes reds, sometimes whites).
From what I can tell, the course is still working on completing some of the blue teepads. I know this is a work in progress, to eventually have 4 teepads per hole -- a true yellow(novice)/red/white/blue set of options, but this isn't done yet. Completion of these holes will also help with a 5-star rating; especially if they update the signs to reflect the new teepads.
The novice teepads are now sometimes "marked" with red arrows on trees pointing to the teepads (similar art on the signs). It is confusing at to whether these are mandos or not. I've learned that they are not intended to be mandos -- just indicators of where the novice teepad is located -- but it's not clear when playing the course.
No trashcans on the course. Seneca is a pack-it-in, pack-it-out park. This is actually good in that locals do a great job keeping the course clean, but it's just something to be aware of.
Another minor con is that -- to some degree -- there is a lack of "signature holes." There are some great holes (6, 10, 12, 13, 14, 22, 26 are my favorites), but there are none that light up my memory and make me dream about them at night like some signature holes other places. Seneca's closest holes to "signature" status are probably 10 and 13 (if you only have time for one loop of 9 holes, definitely play 10-18), but they just aren't quite there.
However, the overall balance of holes and the "fun to play" factor makes up for the lack of signature holes. I'd rather have 27 very good holes than the predicament at courses which have one or two epic holes and a lot of filler.
Only other small cons I can think of: the park opens late (around 8 or 8:30) and closes at sundown, so dawn golf and glo-golf aren't possible.
And: despite being a large state park, there is no camping at the course, which would be just make it that much more awesome.
State park with a $3 fee per person to enter -- a pretty minimal fee if you ask me. I'd gladly pay a lot more to play this course and to support the state park -- it's totally worth it -- but for the sake of people coming to play, it's worth mentioning up front to have a few bucks on you in cash.
Other Thoughts:
The teeboxes are good -- not great -- but good. Plenty of traction and adequate in size, if not always perfectly flat. These are volunteer made and laid paving stones, scuffed for better footing. Some people will be picky, but I like them. Occasional teepads are holdovers from the original course, and are smaller slabs of poured concrete (also adequate).
Seneca should be on your "must play" list if you are in the DC area. This and Patapsco share the crown for top MD courses, and are both A-list courses on a national scale, comparing favorably with the best that any state has to offer.
If you want a course with more epic, memorable holes, go to Patapsco and play the "Green Monster" layout. If you want a relaxing, fun, challenging day of disc golf that will encourage you to try new shots and expand your game, go to Seneca. Better yet, go to both.
Seneca is a course that has grown on me over time. The more times you play it, the more you forgive the lack of "stand out" holes and the more you appreciate the incredible balance and consistent challenge that will shape you as a player and force you to reconsider and improve your mental game as well as just your throws.
If you have a chance to play the Seneca Soiree, held every spring, it's one of the best PDGA Pro events in this part of the country. It's worth coming back for.
The Mid-Atlantic Am Championship (MAAC) is on the short list of best am-only tournaments in the country. Any am who can should try to play this every year. Before I went pro, I played several big Am events, and this and the WVAC (West Virginia Am Championship) are among the best I know of anywhere in the country.
I know Seneca is local to me, and I'm biased, but there are few courses in the country that are as much fun to play as this one. It is beautiful and relaxing -- unlike some tournament-caliber courses that can leave you exhausted, this one is more like a gentle, friendly hike in the woods, that just happens to have some amazing disc golf. Seneca soothes the soul in a way few other courses come close to. I hope you get a chance to play -- you won't be disappointed (unless you don't have a roller, then you'll either have to learn one -- like I did -- or start cursing the cedars). In short: an amazing, one-of-a-kind course.