It was a truly special game in its heyday; there have been many attempts but no successes at recreating what made UO so special. The adrenaline of PvP in that game is something I've only seen matched in a small handful of titles since.
From everything I've read and watched, Eve Online comes (came?) pretty close to the same general feeling/atmosphere of UO but to be honest I've avoided Eve because I don't want to get hooked. I don't have time for an MMO addiction these days.