I've been participating in the Quake Live beta since November. IMHO they've done a really good job of bringing Quake 3 to the web. The game client is slick - just as fast as you would expect of the original. In game, I honestly can't tell the difference. The plugin interaction is quite smooth too, sitting neatly in your browser or running at full screen without issue.
The biggest feature is the whole system they've built around that, though.
To me it feels like Quake Live will make an excellent inter-office deathmatch game. The ability to jump into a game with one click, and have it launch from or live inside your browser is far less effort and disruption than any current alternative.
My only gripe is that they still don't have a Linux client. That pretty much rules out all of the developers at my work - the very same group who are likely to be up for a quick frag. That's a shame, because UT2k4 and OpenArena have never really caught on there... oh well.
I have my fingers crossed.