Pros:
Twin Rivers is located in a park that offers exactly what disc golf courses need: There are heavily wooded holes, holes with danger of losing a disc in the river (aka water hazard), and wide open holes across a field with enough rough bordering the fairway to keep it interesting. None of the holes are long enough for an advanced player to truly air it out, but two of the field holes are slightly uphill and will still challenge the average player. As I'm not advanced, and most people playing here won't be advanced, I won't review it for the advanced.
There is a map by the trail to hole one. Although this map is outdated, it does help to have it for most of the course. Plenty of parking is available and the park also offers softball fields and soccer fields if you want to go out and work on your drives.
Twin Rivers is an extremely challenging wooded course that will challenge most aspects of your game. The biggest challenge will be to your accuracy. The fairways are defined for the most part, with only hole 16 having trees in the actual fairway. The rest of the wooded holes have narrow fairways with plenty of "tree hitting" potential.
Cons:
This course is just too overgrown to play alone. You know the instant that you step up to hole 1 that you're in for it. You essentially throw an S-curve down a very narrow fairway bordered by plenty of trees and underbrush. The underbrush doesn't get any lighter for the rest of the course. Even the four open holes have 6' high grass waiting to eat any errant shots. In the infamous words of Scott Papa, "The best way not to lose a disc is to land on the fairway." Heed Scott's advice and you'll enjoy this course a lot more. If you don't heed his advice, you'll find that the underbrush is a con. I get the feeling that this course doesn't get much play, because the brush hasn't been trampled down any of the times that I've been there.
The teepads are atrocious. They've adopted the "bury some logs in the ground and call it a teepad" approach here. They were dry when I was there last, but they do tend to collect water in the rainy season and to be somewhat dangerous for a run up.
The baskets are all great... Except for the two that weren't there at all. Luckily the thieves decided to leave the pole behind so we had something to throw at. Unluckily for us, they were thieves.
It's finally listed on here as a 17 hole, which is unfortunately true. This is where the inaccurate map by hole 1 will mislead you. Hole number 8 does not exist. There's no teepad, no basket, nothing. The map will send you to the parking lot. If you follow the natural flow of the course from basket 6 to teepad 7 you're fine. But then walk part of the way back toward the teepad and hook down a trail to your left and it will lead you to teepad 9. Don't waste your time looking. 8 isn't there. It apparently went the way of the 7 and 9 baskets.
Hole 13 is a tough one too. I think the tee is right up against the river for a short 150'ish hole. If it is there is no fairway whatsoever. It makes for a fun putter/mid hole, but I don't know if I played it right. Walk back toward hole 12 from the 13 basket once you're done and turn left at the T in the path. Follow that on until you reach a fork in the path. Take the right fork to find teepad 14.
Hole 18 is a short hike away from the parking lot, and that hike will take you across the fairways for 16, 14, 10, and 11.
Other Thoughts:
This course has some serious potential, but it needs some TLC and some more traffic to beat down some of that underbrush. It's a fun course if you bring a spotter and the risk:reward ratio here can be through the roof. I'm just afraid that it's too far from everything to ever fully achieve its potential as a destination course. Without traffic they won't raise the money for new baskets or better teepads either. If they do get it back up to an 18 and actually get some decent teepads and baskets in I would gladly raise my rating... But with a stretch of three holes with no baskets (and one hole in there just gone completely) it's hard to rate it higher.
If you're coming from the Seattle area I highly recommend a warmup round at Silver Lake or Ferguson and also a round at Lake Stevens in order to make a great day of it.