I have to also advise against UO. It is an excellent game (although I know there are 20 people right now ready to say EQ will kick it's ass, it's just a matter of opinion), and can be very fun, but for 2 important reasons, it's not worth it:
1) The game is starting to appeal towards newbies and screwing everyone else. For reasons I won't go into (mainly because only people who've played will understand), it's safe to say that OSI (Origin Systems, Inc, the company that owns UO) is messing up the balance of the game. It's still fun to play, but most of the original fun in the game that existed a year or 2 ago is gone, and much of the game was redone, IMHO, for the worst.
2) The game is very time-consuming, as mentioned by a few of the testimonials above. My friend, who also played the game last year, failed practically a whole half a year's worth of HS classes due to cutting school to play, and since he started again this yr, has also been cutting school quite often. I also did pretty bad in school, but wasn't as addicted. If you are strong-willed, you can probably get away with playing quite a bit in the beginning then being able to play a little bit once in a while, but if not, you'd better be ready to either get rid of the game, or a large portion of your life.