As I'm topping off season 3, I can say on a rewatch the series is quite a different beast. It certainly is far less engaging because, as MiKeD noted, tension is a major part of the show and knowing the outcome it's not as interesting. However, this allows one to focus on the other aspects of the show. Mainly the characters because holy hell they are even more impressive than before.
Every single character (well, besides Marie maybe) is so meticulously crafted and undergoes such a great, yet logical progression it's amazing. I think especially Walt, because it's obvious right from the pilot that he is a self-centered buffoon overcome by hubris. It's amazing really. He changes a lot throughout the series, but nothing seems to be random change for the sake of change, he generally is still the same character he was in the pilot. I love how at the very beginning the show's concept is summed up by a single line, "chemistry is a study of change", and 5 seasons later it still sticks to it.
I think the only episode I enjoyed on a rewatch just as much as I enjoyed it the first time is "Fly". Such an odd, genius episode. The way it's shot is brilliant too, and it's saying something when it comes to a show with such great cinematography.
It's understandable, really. At the time she didn't know they had access to basically, par devastating events, unlimited supply of events. For christ's sake, the operation cost what, 100 thousand dollars? That's a freaking lot for a family that is so cash-starved that Walt's wife who is in a very late stage of pregnancy decided to undertake a job and returned to it as soon after labor as she could.
On a rewatch it's even more mind-boggling how much Skyler is overhated.