My personal experience with the LFD feature is similar to Reddo's. It's been pretty smooth sailing, and the rocky parts weren't that awful. I sometimes thing that the negative press that using the LFD gets isn't as much about the feature itself, or the people who use it, as much as it is about the expectations of the people who don't use it, for whatever reason. I get that the hassle of some of the asshattery that goes on, and the randomness of your companions doesn't appeal to everyone. I go into it knowing that we may not finish, and that there may well be jerks, noobs, asshats, incompetents, meanies, uglies, you name it, waiting for me. I expect them to be there, so I'm not surprised, or disappointed when things aren't perfect.

I'd agree, time of day makes a big difference. Typically runs in the daytime go really smoothly for me. Late afternoons, evenings, and weekends are the pits. I don't touch the Dungeon Finder on the weekend, unless I have a whole afternoon to kill.

On a related note, I've had more headaches from guild runs than I've ever had from LFD. I haven't tanked in Cataclysm, but I have two healers that I geared using the LFD mainly because it has actually been easier and faster for me. You can bail on a LFD group if the experience isn't to your liking, try explaining to your really nice, but not so great, guildy that you really just need to end this now. You can't just bail on them, so you try to help, then suffer in silence until it is all over. Then go queue for the Dungeon Finder, where you know what to expect.