I enjoyed the first two seasons, but I definitely felt like something was missing from it. It's a show that should be better than it is, I think. I listened to a Grantland podcast where they essentially said the same thing... something was missing. They suggested the fact that it was a period piece with recognizable characters might be what holds it back. It really limits how much they can really do with it when compared to a entirely original show. It's still fun to see that era done so well, though.
It also suffered a bit from being predictable. Not just the fact that you know what's going to happen with certain real life characters... but the fact that certain things are telegraphed. You knew Misses Schroeder was going to hook up with the Irish kid within seconds of them meeting. Jimmy and his Mom was pretty predictable too.
As far as it being the best show on TV... I'd have to disagree. I think I'd go with a handful of shows over it. "Mad Men", "Breaking Bad", "Game of Thrones"... as of late I'm really enjoying "Homeland".
Sadly, I haven't watched a single episode of season 3. I think with the climax of Season 2 and one of the main characters getting offed, I've lost a little interest. It's a shame, because that character was one of the only entirely fictional people on the show... which meant they had freedom to do whatever they wanted with him (within the limitations of that world). I imagine at some point I will blast through the episodes on DVR, but as of right now I just can't get myself excited to watch it.
For what it's worth... the last time this happened to me (solid show that I enjoyed, but i inexplicably lost interest in) was "Deadwood". At some point I just stopped watching Deadwood. I think I got through the first two seasons and then just lost interest despite the fact it was really good. I feel like this show has some things in common with Deadwood. This shouldn't be confused with bailing on a show intentionally, though. For instance, I got suckered by hype and peer pressure into watching two seasons of "Heroes"... recognized that it was garbage and jumped ship. "Boardwalk Empire" is good.. I've just been avoiding season 3.