To be honest, Memento is probably my least favorite Nolan film (of the ones I've seen, anyway). I don't mean to say that it isn't good, because it's definitely good, but... I don't know. To me, the story just really isn't all that interesting. As Cool Chris said, it's constructed well and told in an interesting way, but there's nothing particularly special about the story itself. The upside is that the first time I watched it, I thoroughly enjoyed it. The downside is that now that I know the story, the whole movie feels slightly empty to me whenever I try to watch it again - the suspense and mystery feels manufactured and artificial, because I know that the only reason there's any suspense or mystery in the first place is that it's told in reverse order. It's a novel concept, but once the novelty wears off, there's just not a lot of substance left for me.
I know I'm in the minority, though, and that's fine. As I said, I still think it's a good movie. Just, not as good as people make it out to be.
I'll put it this way: it's a movie I admire, but not one I actively enjoy very much anymore.