I bought and played Rift on a whim a couple of months ago.
I thought the interchangeability on the souls was a great idea, allowing for some great diversity in builds that WoW (which is of course the one game it's going to be compared to the most) lost, especially after 4.0 (iirc) dropped talent trees to 30-some-odd points. This also makes for some builds that are just outright sucky but that's a small price to pay for a comparative lack of homogenization. A downside I was experiencing to this was that having never gotten past level 20, I wasn't able to delve deep enough into each soul's tree to really feel the impact that my soul choices were having, both in terms of what souls and what talents I took in those souls.
I like that each form of Rift has its own style of portal and mobs. It's rather intimidating to approach a Water Rift and see a massive, tentacled beast loom above you. The idea of "global quests" was something I really liked in Warhammer Online, so the Rift system was a plus for me in that regard. That your rewards for closing a rift scale with your participation gives everyone there motivation to pull their weight, and was a great (if perhaps obvious) choice on the part of Trion. On the flip side, if a high-level character (even one just five levels above you) decided to step in, your comparative level of participation would drop through the floor and you'd get little, even if you stayed start to finish.
In the end the game felt "hollow" somehow. I can't describe it, but having played WoW off and on for three years I was almost getting that "uncanny" resemblance issue some people have with incredibly realistic CGI characters; it's almost bothersome how similar they are in gameplay despite their nuances, and my liking WoW quite a bit didn't help when my one month of gametime drew to a close.
I'll definitely be keeping an eye on this game, though. Given some time to mature it could become a great MMO.