Other than Diablo 2, I've never really played any of these mmorpgs. I'd be really tempted to give this one and Fallout a try though! I doubt I'd ever find the time for them though. I really wish they'd make another single player KOTOR 3, using a similar engine as KOTOR 1, 2, or perhaps the new Fallout 3 engine.
In the meantime I'm just anxiously awaiting Fallout 3: Vegas and Fallout 4 which supposedly (I highly doubt) is coming out in next spring.