I have 5 bikes; 2 with discs. 1 FS mtb bike and 1 hardtail with slick tires used for commuting. Both have mechanical disc.

Mech Disc brakes don't necessarily stop any better than rim brakes. I had Tektro disc brakes on the hardtail and they were crap. I replaced them with BB5s. Much better.

I have BB7s on the FS mtb and they are better still than the BB5s.

I have Dura Ace dual pivot road brakes on my road bike, and they are better than the BB7s. The weigth differential is only 15 (ish) pounds.

Yes, the discs can squeal, especially when wet and this can be alleviated quite a bit by using organic or resin pads.

The discs offer better stopping ability in the wet as compared to a wet rim. Rim brakes, when riding in muck off-road, will wear down rims pretty fast.

I find discs much better in the winter, they do not ice up (never for me anyway) like a rim brake would.

For off-road, I'd recommend a good quality set of disc brakes, but you'll pay a pretty penny for 'em. Don't buy the lower end disc brakes and if you have to choose between low end discs and a set of rim brakes - go with the rim brakes.

For on-road, I see no real advantage with disc brakes with the possible exception of heavy loaded touring or mountainous regions. But I tour on rim brakes (Avid shortys with STIs) and have no stopping issues.