Having bought Tekken 6 on the day it came out, and reading many of the reviews released for the game, it appears that the game is struggling in terms of offering a stable experience while playing over Xbox Live.
Now if you have not guessed already, I bought this for the Xbox 360, so that is what my impression is based off of. But I have played about 12 matches over Xbox Live so far, over half of them being great lag matches. The problem with fighting games and lag is that it requires a stable and smooth framerate, or you simply will not be able to execute many of your combos or juggling attacks, because they are based on timing. So when you enter a lag match, you will pretty much just be button mashing hoping for a combo to come out. Most of the matches you may encounter will most likely be a lag match, and I know that this is not just my connection, since I have a smooth online experience for other Xbox 360 games. You can also read reviews of the game on other websites, they most likely will have the same critique of the game.
Now, there is another thing I noticed about the game while playing online, though some of you veteran Tekken 6 players may be used to it, is juggling and turtling. People who are considered pros at this game will often just sit back and wait for you to throw the first punch, and when you do, they will counter with their move and catch you in a massive juggle combo. Then they will rinse and repeat. So yeah, a players first reaction to this statement may simply be “get better at the game”. But a first impression is important, and being caught in juggle combos that you are not able to even get out of probably alienates and bores more gamers than just me. I mean, if the combo is a legit and flashy combo, I will not mind so much, but when most of the players I encounter are simply cheap and turtle (constantly block and wait for you to make the first move), then catch you in extensive juggles, things start to get boring.
This is my general opinion of the Tekken 6 online play. But honestly, this is to be expected of the game, since juggling is a mechanic that has been kept in the Tekken series for quite some time. So basically, for those of you who are already fans of Tekken 6, you will not have a problem with online play except for the fact of constant lag.