I'm a big fan of Terry O'Quinn, so I thought maybe 666 Park Avenue would be worth checking out... I've seen the first 5 episodes so far, and I'm not too impressed. Terry O'Quinn is great, but his character seems poorly written to me... he's an all-powerful Devil one minute, then he's just as scared and confused as everyone else is the next. And several other characters seem to have a serious case of the stupids, which can be really annoying.
The whole show is a bit all over the place, really.... so many different things are going on at once, but hardly anything ever gets resolved in any meaningful or satisfying way in any single episode. It's a bit like watching a fantasy version of the X-Files, but one that's way too heavy on the "mytharc" stuff and way too light on the standalone monster episodes. And there's a lot of random weird shit happening that doesn't seem very connected to anything, such as the evil suitcase that contains the smoke monster from Lost, or something. This makes the pace of the show feel very, very slow... it takes about three episodes to give you one episode worth of meaningful stuff actually happening.
Also, a recurring theme of the show seems to be that money is the root of all evil... I think that would really start to grate on my nerves after a while, but your miles may vary.
Update: Well, the powers that be over at ABC have decided to pull the plug on the show, so I guess I can stop whining about it now. I just hope Terry O'Quinn ends up in something better next time... his talents seemed a bit wasted on this, in my opinion.