I found season 1 was meh, and when they were still just copying The Simpsons. S2 was alright, and S3 was good, and the first half of season 4 would be my favourite (up until The Fat Guy Strangler). From there I've found they've sort of flip flopped between good and bad seasons.
The problem is that the characters are simply there to support the jokes, instead of the jokes forming organically from the characters like it did with The Simpsons, and that's why it doesn't hold up over time. But I don't even mind the shoehorned cutaways, when they're actually funny. It's the overly drawn out ones, and the pop culture references for the sake of references that lose me (yeah yeah, someone's going to say that's all of them
).
I still enjoy new FG, despite it's problems.