I don't know how accurate/reliable it is, but I found one source quoting approximately 135,000 subscribers as of May 2006 with 70,000 of those being in Japan. It also said UO's peak was in 2003 with around 250,000 subscribers. Again - I didn't put any effort into that research so I don't know if it is reliable info or not. I was surprised because I always thought UO was somewhat of a leader in online RPG's until the last couple of years.
As far as gameplay - I tried WoW and wasn't impressed. It just felt like any other fantasy video game where you started out and quests led you on your way. I felt like you had to follow the prelaid quest plan to advance and didn't have much room to go your own way. The graphics were definately much more impressive, but I still like my 2d UO. I haven't found anything else that offers such an open world with such an overall variety in what you can be, what you can do, customization (houses and characters, etc..) - and the community interaction seems to take place so much smoother on UO than anything I've seen. Everyone has their own preferences - but UO will always be my 1st.