Great software. Clean look. Large pool of players, so you don't always run into the same ones. Excellent and prompt support.
Cons:
Tourney schedule is a bit rigid.
I started out on Party, but immediately got fed up with the cartoonish look and feel, and the ridiculous play of the hands, where monsters were dished out like candy. I absolutely believe that Party does not deal actually random shuffles, pumping up big hands to increase action. Poker Stars, on the other hand, is a class operation all the way. Great software, what I believe to be truly random distributions -- the game just plays like a real game, not a video game. The Stars RNG has been tested and retested by reputable testing organizations and found to meet every quality standard. The support staff is responsive and gives comprehensive explanations to answer all inquiries. The competition is tougher than Party, but for me, that makes it all the more enjoyable and challenging. I believe that if you give Poker Stars a try you are not likely to look to any other site in the foreseeable future.