I've never watched this show before recently, and I'm currently halfway through season 5.
I must say that seasons 3 and 4 in particular are remarkable, and season 5 has been very good too, but there's a fucking HUGE tonal shift in the "Stress Relief" episode, which I learned was aired after the Super Bowl to get as many new viewers in as possible, and the couple episodes I've seen after that one just don't feel the same. It's still very good but... I dunno, I can feel myself becoming unenthused about it. And I'm certainly not encouraged by the comments of others here detailing its decline!
I'm most dreading the switch in Andy's character in the later seasons, since I've grown to love him and he's now probably my favourite character in his current stage of being adorable and utterly stupid. I know his switch from knobhead to nice guy was quick but I didn't really mind it at all, I found it believable enough.