Fun, up to a point. I've been playing this game for a month and it's no replacement for some of the older, obsolete, or other online versions I've played on other systems, but this one has the best graphics and (sort of) response.
Randomness is a major issue, as seen in other comments and links to other forums below. There is a difference between true randomness and the illusion of randomness. Also, the randomness changes to a more "difficult" randomness as the game progresses into harder levels. I've yet to be able to pass level 7 because suddenly I'll have 5, 6, or 7 of the same S and Z pieces thrown at me, and I'm definitely not THAT good to figure out how to get them all pieced together. Also, once I reach level 6 (I fully understand, btw, low levels for someone playing Tetris as long as I have), the pieces just fall and I can't move them fast enough and eventually the game just ends. I can "dodge" a locked tetromino for a while on lower levels, but not so on these higher levels.
I'm not sure, but I think every Tetris I've played has a different "persona" and around levels 6 and 7, this game just becomes a nuisance and annoying. No matter how often I play, I cannot get past these levels without being handed the same tetromino over and over and over. It's very frustrating. This game is otherwise almost perfect. So close.