I like the medic. He's a team player, and invaluable in every game mode, and he can hold his own in combat if you've got the Blutsager equipped.
I want to like the spy, and I'm getting better with him, but I'm not that guy (you know that guy) who can take out an entire team with a spy. I can get a few kills and be a nuisance, but I'm not a one man spy army. Yet.
I'm confused as to why the Demoman, the guy in charge of indirect fire and blowing shit up, is now the most viable melee class. As for the engineer... he needs more options.