Since Star Trek Online went free-to-play, I downloaded it a couple weeks ago. I've had mixed reactions to MMORPG's in the past, but I figured I might as well give it a try since, y'know, it's free.
After playing it for a couple weeks, I must say that I am truly impressed by this game. Lots of variety and customization options help prevent it from becoming too monotonous. There's also some good incentive to keep leveling up, since at certain levels/ranks you gain access to new ships and new duty officers. Now obviously the creators are in the business to make money, so you're encouraged to spend real-world money on "cryptic points" to unlock custom items/uniforms/etc. in-game, but you don't really need to do that in order to have a rewarding game experience. It's totally optional, and choosing not to buy any "cryptic points" doesn't in any way detract from or substantially limit the game.
Whether or not you're a trekkie, if you've been waffling on whether or not to try this, I'd definitely recommend it. It's not like it costs you anything to give it a shot, right? (Aside from lots of potentially squandered spare time, that is...)