Played it for 3 hours. It is definitely a fun mod, but doesn't feel like TF. Dave lists some good reasons, in my experience every class but scout feels like you are controlling a fat man. After 3 hours as Soldier I was able to rj through the hole in 2fort4. Soldier works better as defense than offense because it is largely an immobile class. You'll notice Soldier is listed as offense because usually they could move around the map pretty fast in their own respect, not really true now. I managed to make some offensive plays, but it was usually when the entire other team wasn't watching middle.
Sentry guns are friggen insane now. - Engineers are 1 man wrecking crews because of the lack of grenades. Even if you know where their turrets are and nobody is even defending with them, they are very hard to deal with. Which is an awkward scenario, turrets shouldn't be build and forget.
I was shocked at the amount of melee, melee kills in old TF were something to be proud of, but they happen around every corner in TF2. I saw scouts getting people on their dominated list with the bat.
As far as defending, I fought A LOT of spies. Most popular class for the entire 3 hours. Mainly because they are the only class with a decent shot at getting past turrets which makes them the premier offensive class. Scouts were pretty popular, but I was surprised to see the 3rd offensive class as *drumroll* HW guys. Hey, shouldn't you be back at base defending you freaking tank. 1 HW guy and a medic plowed through a whole bunch of turrets. But for the most part me and a demoman kept them at bay.
Firing the rocket launcher in TF2 is equivalent to firing the rocket launcher in Quake1 with a 400-500 ping. Aim way ahead. It took me about 15-30 minutes to get the timing down good, but I noticed a lot of players almost expected you to miss, when they saw soldiers they'd keep running at or around you with the assumption that you'd miss. Once my aim got decent they'd step into the fire most times, but the feeling of having a sluggish rocket launcher never really disappeared completely.