I've got lower-end Shimano hydraulic discs (M486s, I think). I love them. I specifically sought them out because I ride year round, and loss of brake power due to excessive ice/snow buildup on rim brakes is a significant concern. Even in nice weather, the control and power is excellent and noticeably better than rim brakes I've used (granted, I haven't used a lot, and most were relatively low-end). I've had them about a year, and haven't had to do any maintenance as yet either. By and large, They Just Work.

That said, occasionally during the worst part of the winter I did manage to get enough ice and snow buildup to degrade their performance. I could still stop, but there was a noticeable reduction in friction between the pads and the rotors.

I like them enough that I wish they existed for road bikes, and I'm putting a set on my Mundo cargo bike.

Don't get me wrong, for the majority (even vast majority) of riders V-brakes are going to be just fine. They will work well enough in most weather conditions. But if you want something that will stop you right freaking now no matter what the weather, and give you precise control of your breaking power, it's hard to beat hydraulic discs.