How long to people find that their discs and pads last? It will allow me to do a comparison.
It all depends on the conditions you're riding in. I had a disc braked cyclo-x bike for a while (Planet-x uncle John). In the 6months I owned the bike I never had to replace the disc pads and I was doing well over 1000miles a month on it! Yet, on the MTB I've destroyed a set of disc pads in just one ride before now.
Generally speaking disc pads outlast v-brake pads a good 5-10x over so you should get a few months use out of a set.
Also how well do these brakes perform compared to rims and pads on road?
It completely depends upon which make/model of brake you've got.
As you go up in price not only do the brakes become stronger they have better modulation too.
Cheaper cable operated discs are stereotypically rubbish and should be avoided.
The more expensive cable discs (aka avid) work very well but are fiddly to set up.
The very cheap hydraulic disc brakes tend to be lacking in power or they have an on/off nature which makes it all too easy to lock the wheels.
By the time you get to the level of Shimano Deore hydraulic discs you can't really go too far wrong, they all work very well.
My next bike will have discs, is there much difference between hydraulic and mechanical disc brakes? Mechanical sound like they will be easier to maintain and fix on the road should anything go wrong.
Most hydraulic brakes are fit and forget and rarely go wrong. Apart from changing the pads every now and then you probably wont need to touch them. Cable discs tend to require 'tweaking' as the cable stretch/pads wear.
O/T Disc brakes do make the wheels of the bike heavier and the rotors are prone to rubbing. This means that bikes fitted with discs are often slower on the road than those with v-brakes.
Off-road, the advantage of being able to stop quickly and in all conditions more than compensates for the weight penalty and ideally if the brakes are correctly set-up the rotors shouldn't rub.