Hopped on for a bit today... So here are some initial impressions.
Tons and tons of spies! xD While a good spy-checking regimen will keep them at bay as usual, they will never be caught in corners trying to hide and recharge anymore, so no more randomly coming upon spies and getting a free kill. The rechargeable cloak means that if a spy gets away and cloaks, they'll have a much easier time of coming around from a disorienting angle than before 'cause they can cross more distance more quickly.
The nerfs to the Demo aren't too bad honestly, as the last thing you'll be doing in the heat of battle is blasting at his stickies. It does make him useless at separating ubers, as Gopher mentioned, but then the Pyro basically becomes the anti-uber specialist because he's got compression air blast juggling... I have seen 3 simultaneous ubers wrecked by a crew of Pyros, kind of insane. Makes the class much more valuable on defense, as he has something to do other than spy check incessantly now. :P
Engy changes really increase the pace/frenetic nature of a match. You get a level 3 tele or two and you'll have people on the front lines as fast as they can spawn. The charge time is almost instant. On the other hand, I think that the Lv. 3 dispensers promote turtling to an insane degree. Sentry-based defenses now have legitimate stopping power instead of being a nuisance that an offensive class + Medic could take out without too much trouble.
More info when people call Medic is hella useful, lets me know who the priority targets are and who I can ignore. :P
I enjoy the changes quite a bit. So, plus points to Valve for successfully making more classes viable and useful with this patch.