TF2 is an all around great game, and a must-have. It will appeal both to hardcore FPS fans and casual players. It's great fun.
You can pick up The Orange Box (includes Half Life 2, Portal, and TF2) for $10 on Steam this weekend - give it a go. I have no doubt you'll find it addictive and hilarious.
EDIT: Yeah, TF2 does have a bit of a learning curve. If you already play other online FPS games your reflexes should be fine - it's more about learning the roles of each class. If you have trouble at first (some classes are somewhat easier to start out as) get on the TF2 Wiki (google for link) and click your favorite class. Scroll down and there should be strategy tips, strengths/weaknesses, etc. It helped me a LOT. Also, I found it much easier to learn the maps when I knew them by name. If you're going "gee, what map is this??" open the map info (i by default) and check. It's confusing at first because some maps have several stages.
Overall, still, it's an excellent game. Humorous, fast-paced, and fun. I can't recommend it enough - I get more fun out of that game than half the rest of my gaming collection combined.