Originally Posted by TallRider

My opinion is consistent with what others have said, but I'll try to add some explanation (and a couple more points).

1) The talk of disc brakes being obviously stronger in good weather is largely marketing hype. As others in this thread have noted, discs aren't "better at being brakes" in dry weather. The increased clamping force of discs are canceled out by the much smaller disc diameter compared to a rim.
2) Disc brakes are generally better in wet/mud for two reasons: (a) the disc is further from the ground, and doesn't get wet or muddy just based on what the bike wheel is rolling through (b) wet rims need a revolution under the brake pads to scrape the water off for better friction between rim and pad. Disc brakes have higher clamping force, which squeezes the water out from between pad and braking surface almost instantly.

Other relative upsides of disc brakes:
3) you can easily swap between different wheel sizes on the same frame
4) no need to replace the rim once the braking surface is worn down. this matters more for commuting bikes (where braking is more frequent and rims will thus wear down more quickly)
5) easier to use with fatter tires and/or fenders, where longer arms of normal sidepull brakes will mean more flexing and less braking force (this advantage disappears when comparing disc brakes with centerpull rim brakes)

Other relative upsides of rim brakes:
6) less finnicky adjustment when changing wheels, because tolerances are larger
7) a flexible fork (which gives a smoother ride) can be used, whereas disc brakes require a stiffer fork.
8) pads are less likely to "fade" from overheating on long descents, because large rim (esp aluminum) acts as a better heat sink than small steel disc

All this means that disc brakes are obviously better for off-road usage.
For road riding, things are much less clear.