FOB used to be pop punk and emo-pop (or something similar). Now their latest two albums consist of a wider range of genres, including pop rock, electronic rock, even R&B. Let's face it - they DID become more mainstream and most people liked them better when Patrick was "fat". I personally think that their old music was more unique but their new music is catchier. I love both :)
I love the old FOB, but unfortunately they have sold themselves to pop now. It's just what's selling right now. I mean, they still have an amazing sound- no one can deny that Patrick Stump's voice is one of a god. It's just not the best as far as actually style goes. I like both, I just rather them produce music like their old stuff. I know, don't hang on to the past, they've moved on, but still...
I say old Fall out Boy because I really got into the band before their hiatus, and their new music borders on pop/electronic at points, which I'm not very fond of.
They was a decent pop rock band at first, there first three albums are slightly enjoyable but now they are just a bad pop band trying to keep there fortune.