TF2 is awesome, the pacing on the combat is slower than most, which I love (isn't twitch based gameplay). The nine classes of characters are all balanced well with advantages and disadvantages to them all, meaning that although the guy with the Minigun can kill lots of people, because of his slow speed he isn't likely to complete any objective before he dies (unless he teams up with a medic).
The graphics look good and surprisingly the game runs well on my system (GeForce6, 2GHz, 1GB) which is great because when I tried the Quake Wars Demo last week, even with all the settings off and on low, the game still ran just above unplayable.
Before the Beta was launched there was some concern as to wither this was a true Beta or just an exclusive demo as there is only a month left before the game is available for purchase from retail. But no, TF2 is most certainly a beta, the game has lots of stability issues and the TF2 Forum is filled with people having issues. I myself haven't finished a whole game yet as it keeps freezing and crashing. I recommend running TF2 in window mode instead of fullscreen so when the game does freeze it is easier to get to the Task Manager.
I wasn't expecting much, and I only bought The Orange Box for Portal, but I have been pleasantly surprised.