I've been hearing more complaints about the lag than not. I was actually trying to save up just to get this game on release, and sadly I couldn't hold the money. Anyways, I'd have to say I'm glad I didn't buy the title right away. The complaints about lag seriously seems like you can't even play in a 4bar connection. A lot of these complainers, will say to me that they can't finish up a juggle combo because of the delay.
I have also heard that it's not that bad, and that it's playable. Considering a lot of the SC community, at least on my friends list, is playing T6 all the time- I can't say it's horrible seeing as how they're still stuck on it. I'll be waiting to get my hands on T6. People are comparing the lag to how it was when SCIV first came out. (Apparently there was a horrid lag to begin with) So, I'm going to wait it out.
However, I've played Tekken since Tekken 3. I'm not a huge fan, but I've kinda played here and there. It's fun though, and maybe I'd mainly play it offline instead of online if the lag is really that bad.