I think it's a great show but they are definitely on a Simpsons-style progression curve, where plots are going to become increasingly more implausible and characters become more and more cartoonish.
DeVito does not ruin the show, but at the end of the day he seems more and more superfluous. He doesn't really drive though show, and when they devote plot threads to him it's usually just boring. The classic things that people seem to remember about "It's Always Sunny..." never revolve around him. The writing is good enough that he isn't needed to draw viewers, but if a big star is what it took to keep the show alive during rocky times then I must salute him all the same. The problem is that they don't really need him anymore, but are obligated to keep him around and use him frequently just because he's probably drawing a huge paycheck.
