I stumbled upon It's Always Sunny in Philadelphia shortly before its 3rd season started (which is good, because I have to say that its most recent season was its weakest) and am pleased to tell you that this show is comedic gold.
It is definitely one of those offbeat sort of comedies - for people that like the likes of The Office, 30Rock, Arrested Development and Curb Your Enthusiasm - not that it is really very similar to any of them.
The show revolves around a group of friends that own and operate a bar that basically are all terrible people. Bad people make for good comedy though, and the show manages to perfectly straddle the line between being flat out offensive (with episodes devoted to abortion, drugs, molestation, etc.) and just being hilarious.
It's Always Sunny is "very talky" to say the least, and frequently just ends up with all of the characters screaming at each other, which is obnoxious to some I imagine, but worth putting up with. The show reminds me the most probably of Curb Your Enthusiasm, in that each episode builds up a single theme for the duration of the 30 minute episode that always culminates in something that is just beyond ridiculously hilarious.
Give it a chance (and I recommend Dennis and Dee go on Welfare if you only want to watch one) and you won't be disappointed!!