Definitely worth a try.
This is definitely one of my favorite games to play when killing time. I can't think of very many negative things to say about it. So lets start with the positive.
The class system is one of the best things about it. It's fun having 9 different play styles to choose from. My personal favorites are Pyro and Spy. It's also great to have a game where someone like the Heavy can actually be killed. The unlockable weapons are a very nice touch, and there's always more. Strange weapons are a must, if you can get your hands on one.
I love the multiple modes of play, and that you actually have to try and work as a team to win the round. As far as hats go, when you inevitably get them, try and not trade them if possible. I once traded away an Attendant and a Strange Shotgun, and I will never forgive myself for that. Crafting is time consuming, and requires that you have tons of items, so I wouldn't reccomend trying it until you get a lot of random drops. Although, when you do craft that item you've been wanting forever, it feels rewarding.
Custom content is another thing that I like about the game. As far as mods go, I personally love "Vs. Saxton Hale." One person on the server randomly becomes Saxton Hale (a character from the TF 2
comics) at the start of a round, and the rest of the server hast to kill that person. Essentially, it's a big boss battle that everyone has to work together on.
The community is one thing that can be negative. They tend to bitch and moan about every update. But if you look past that, it's a really fun game. Since it's free, there isn't any real reason not to try it. And it is definitely worth at least a try.