I've just been playing the beta, and I can report that it's great fun.
It's significantly different to past incarnations of Fortress; there's no grenades, and the controls have been streamlined and simplified.
Note that by simplified, I don't mean dumbed-down.
Anyone who's played previous Fortress titles has to admit that all of the binds you needed to be efficient could get a little overwhelming.
The classes all have unique personalities, and these personalities manifest while playing.
Expressions change, comments come unprompted and it all ties together with incredible style.
I think this may even have the power to pull me away from GW:EN, it's that compelling.
That's not to say it's without problems.
Mouse input for me seemed to go a bit haywire at some points, leaving me without control.
I had the game crash whilst trying to connect to a server.
The very first server I joined, some joker had set up a killing zone outside the spawn point and had changed server damage variables so that you died immediately on spawn. I suspect this was in an attempt to artificially inflate his stats.
So still pretty much beta, but not problems it's going to take Valve an age to iron out.