I tried it briefly, based on a friends enthusiastic recommendation.
I have to say I feel like I wasted my money though.
I've played MUDs since I was in middle school (I'm 26 now), I've played MMO's since Ultima Online, and I've done my time with FFXI, WoW and Eve Online, and have had brief stints with smattering of other MMORPGs, not counting countless hours of Diablo 2.
Rifts does nothing radically different. Yes, it breaks from traditional class system by letting you pick multiple classes to work with, but its not a new idea, and it seems to unbalance things terribly.
The big problem, for me especially, is that the base Operant Conditioning reward formula remains unchanged. For me, it was interesting in MUDs, and I have become progressively more and more burnt out on it as each iteration of the same has returned. Do boring tasks until you receive a reward, which you use to do more boring tasks to get more rewards and so on.
I put those games in a class of games I like to call existential questioning games, because I find myself questioning my existence if I play them too much. "Lets say, I've got all the levels, all the best gear, cleaned out every dungeon, have every raid on farm, solo every rift, defeat every other player with minimal effort... yet I've learned nothing significant from the process except a relatively small amount of information which only applies to this game, havn't challenged myself except in my capacity for patience, and outside of the game world, have nothing to show for it... Whats the point?" I can't even apply the argument of intrinsic entertainment to the game because I am not being intrinsically entertained, I'm simply being rewarded for tasks unending.
Contrast this with Portal 1: I finished that game feeling downright satisfied, like I've learned a new way of thinking, like I challenged my comprehension of physics, and that I was genuinely entertained by the story.
I would contrast it with an MMO that "gets it right" but as far as I know, there isn't one. There are a whole bunch of reasons why that is, but whatever.
tl;dr
I didn't like it. This is why:
http://www.escapistmagazine.com/videos/view/extra-credits/2487-The-Skinner-Box