It depends: why didn't you like vanilla TF2? What do you think you'll like about this?
Vanilla TF2 was 1000x better as an actual team vs team FPS game. It was focused, balanced, refined and had a lot to learn in the higher-end.
F2P TF2 I'd say is 1000x worse when it comes to the actual no-frills team vs team 'core' gaming elements, however there's plenty to do if you're casual and not interested in learning the fine-points of a class or weapon. Plenty of 'fun' game-modes, a much wider variation in maps, lots of weapons and hats and misc items to keep the stamp collector's happy.
Quite honestly, I cant even remember why I didn't get into it, I might have been more into Dawn of War at the time. But it just didn't grab me enough for me to keep playing, I couldn't tell you the specifics because it was so long ago.
I just figured I would give it a shot as I don't mind playing coop against bots, I enjoy ME3 MP enough and this seems more involving than that.
Downloading it now anyway so I guess I will know soon enough.