It's something that sucks me back in for short periods from time to time now. I got the game about three days after it came out, and I played it every day for hours, for maybe a little over half a year before I initially got bored of it and moved on to other games. That was just with the vanilla game though, the only mods I used then were UI mods. I also only had one character, who became the generic jack-of-all-trades character, and those are no fun to play.
Then I slowly discovered rping and armor/weapon mods, immersive mods and stuff and started experimenting with different kinds of characters.
Personally, I still feel like there's only a finite number of things to do in Skyrim before I get bored again, so like most people in this thread, I play it on and off. It's very easy to get back into, though, and I like that about Skyrim. At the moment, I think I haven't played it in maybe half a month, but there is a build I've been thinking about trying lately, so I'll probably be picking it up again soon.