I've thrown Lat and Westside drivers almost exclusively since December or so; awesome, awesome discs, and their lineups provide you with a lot of possibilities.
Underworld/River/Stag constitutes the main portion of my drivers. I've been throwing the Stag and Underworld since they came out in March, and could not be happier with them (especially the Underworld). I recently added heavy Opto Rivers for a stable driver in between the two, and it's working out perfectly. They compliment each other well stability-wise, have similar feels in the hand, are in the same speed range, and all prefer that characteristic smooth throw. Switching between them could not be easier; they're natural compliments to each other. Between these three, I have all I could ever need in my mainstay fairway drivers.
{I've opted to continue throwing Banshees as my seriously OS disc for a few reasons: I'm already familiar with them, I like how they beat in in base plastic, and I like having a base plastic disc in my bag for water holes/risky shots. They do their job perfectly, so I've had no need to replace them.}
The Underworld is a very understable fairway: as glidey as the River, but with a lot more lateral movement. I can't imagine an easier hyzer flip to turnover disc, and certainly haven't thrown one. Great for BH turnovers, huge annys, rollers, and low ceiling & low power shots. I don't flick, and this disc opened up a ton of lines for me.
The Stag is a great, underrated disc; many were disappointed when it first came out that it wasn't quite as OS as its flight ratings suggest, and I think it gets an unwarranted bad rap for that. I love how much the flight varies in different weights and plastics: light Tournaments fly like glidey Eagle Xs (bit of turn, big fade) and beat in like them too (lose HSS before LSS), while max weight VIPs fight wind very well and won't turn unless you really, really crank (OAT) them. I carry both varieties and they cover a lot of lines for me. Great moderate wind disc: stable enough to fight through while still maximizing distance through that glidey Euro magic.
Sounds like you already know the River. I initially threw Test Material and light Gold Line Rivers, and was disappointed that it wasn't more understable; gave them up for the Underworld. Now that I'm using them in Opto in a different role, I'm falling back in love with them. It gets a lot of hype, but it's warranted.
Also, Euro plastic is generally considered to be finickier than many other brands, requiring smooth throwing motions and good technique. Throwing it has done wonders for my form. Having returned to the River after 6 months of only throwing the Stag and Underworld, I can really see how my abilities have progressed in the River's flight and distance as compared to what I was getting at the beginning of the year. One of the many perks of throwing Lat drivers.
I've tried many other Euro discs at one time or another, but these three have each been in the bag for 6 months or more, with many more likely to come. Can't say enough good things about 'em, and Lat/Westside in general.