I've only had the chance to play TF2 a bit the past few days but it's amazing so far. Everything is balanced fairly well, though I have some issues with it (mostly changes from TFC). I don't care for the Heavy. He seems too easy to kill right now and his mini gun is horribly innacurate. I tried taking down a sentry gun as one and was killed from full health in about two seconds. Just about any other class would have faired better. I also don't like the lack of off hand grenades for every class, though I'm sure I'll get used to it, even if the Scouts double jump has nothing on Conc jumping.
Thankfully there are a lot of good changes though. Infections are gone, Pyro's are actually useful, Medics are more useful and Spies have the nifty ability to take down sentries. Through it all though, I think my fvourite part is that Snipers are largely untouched. Taking head shots with the old red dot was like coming home for me.