I used to only play on Poker Stars. Stayed loyal to them for probably over 10 years, but then it seemed to change. The rewards now take forever to get, and are not nearly as good as they used to be. I'm also finding that insanely ridiculous, runner runner type bad beats, that ALWAYS reward a player who pushes all in, AFTER a preflop re-raise, and with a TOTAL GARBAGE hand, have increased DRAMATICALLY. And yes, I know bad beats are going to happen anywhere at times, but that particular situation seems to happen very often on Poker Stars now, where it seldomly did before. I finally ended up cashing out and refuse to put any more money on their site. I will still play free rolls, or home games, and play with the money won from those, but it just doesn't seem worth it to me anymore.
I opened an 888 account about a month or 2 ago. So far it seems ok. Wins, losses, a few bad beats, a few lucky catches. Definitely has less players than Poker Stars. I find there are a lot of fish on the site, but that's going to happen anywhere. I do sometimes have trouble finding a cash table that has enough players for me to want to sit at, but it depends on the time of day, and there always seems to be something I can get into. Free rolls are alright, it's actually nice not to have every single one filled with 10,000 or more players.
I also just signed up at ACR 2 days ago. I haven't deposited money on it yet, but will when I'm a bit more familiar with it. They seem to have a nice amount of free rolls, and I like that they have those $10 on demand tournaments where you can play without making a deposit, to win a small, up to $2.50 in real money (usually with under 500 players). This is giving me a chance to test out different types of games/tables on their site before making my deposit. I can see myself playing here more often.
I have yet to try party poker. I did sign up, but can't play any free rolls without 1st making a deposit, so since I can't test it out that way (I know I could use play money to try it, but it just doesn't feel the same to me), I'm in no rush to make a deposit. Probably will at some point, but not anytime soon.