I've "beaten" it; I use the quotation marks because some Mario die-hards would not consider completing the final "fight" in the game as actually beating at, and would, instead, point to the notion of getting all 120 stars as truly beating the game.
I have done the former, but not the latter (I am in the process of getting those 120 stars. I just past 100 before my finals period took over my free time).
I have to say that I haven't enjoyed a platformer this much since Mario 64 was released - it just feels fresh and, as a few reviews have touted, doesn't feel like it relies solely on the image of Mario to make the game fun. Brand loyalty didn't make this game more fun (lest it fall to the level of Mario Sunshine - the little platformer that couldn't, in my opinion) - it was just put together very well. I will admit that it was a little on the easy side, but the variety of levels to explore, as well as some of the more challenging, optional stars (those required to get you up to 120) balances that out (adding the requisite tedium that all platformers I really enjoy seem to have).
Even if you are disenfranchised with the Mario series, I'd recommend at least renting the game - you, in any sensible right, shouldn't/won't be sorry.