On my mountainbikes I've had all sorts over the years:
On the front of my old Saracen there were normal cantis, which had little power or modulation. The back had a 'u-brake' which was as rubbish as normal cantis, but also located under the chainstays where all the mud seemed to end up.
My Cannondale ST had cantis on the front and an odd Suntour Rollercam system which was kind of better than v-brakes, and predated them too, but never caught on because the braze-ons had to be in a non-standard position. This was actually a pretty good brake – too good for the back wheel in those days, except for the fact that it would also clog with mud if you went riding in sticky clay (but that's true of any bike to a greater or lesser extent, yet strangely doesn't affect my front wheel on the Lefty fork!).
My Rocky Mountain Element had Vs at both ends - they offered better braking than cantis but were useless in mud or wet, which is a major flaw for anyone mountainbiking in the UK. Not to mention the rim and block wear! Even in the dry though they were not powerful enough: I remember chasing a couple of lads on full sussers with discs across the alps. I got to the top first every time but simply couldn't hold the speeds coming down because I lacked the stopping power, and my forearms and fingers hurt more than my legs by the time I got down. In summary, Vs are ok for light duty but not the best for serious sport, and as someone posted earlier, they can be too abrupt for beginners - some 'granny' bikes came with Vs, but they had a cam system fitted to make locking up the front wheel more difficult.
I also had a Canyon (German brand) which had the legendary Magura HS33 hydraulic rim brakes that MG mentioned above. These really are the dogs danglies of rim brakes, but sadly the usual caveat of grit and mud and rim/block wear applies. Fantastic brakes though – made me reappraise what rim brakes were capable of.
The Cannondale Jekyll has got Magura Marta hydraulic discs. Again, only 160mm rotors, believe me that's enough for most trails in the UK on an XC/All Mtn bike - only DHers with heavy bikes on serious trail centre courses really need more. The modulation is spot on, if you want to launch yourself over the bars you would have the power available, but they offer so much feel you just wouldn't - even in a panic. Servicing-wise it's not a huge consideration - yes, good disc brake systems aren't cheap to buy but running costs I would say are comparable - a disc lasts for years unless you bend it, a set of pads lasts longer than a set of v-brake blocks, costs about £15 and doesn't need fettling. No cables to replace, etc etc so are easy to maintain. Plus your rims don't get ground away every time you use them in bad weather.
I did have a problem of mine going spongy once, but got a competent LBS mechanic (who I happen to ride with!) to bleed them and they're like new. Only cost £15 for the bleed. I did once bend a disc in an accident, Magura discs are a bit dearer than Shimano ones (£35 IIRC) but even this was cheaper than the rim wear I used to suffer. Discs therefore are well worth the money if you're serious about off-road, but are a bit overkill for road.
Below is a pic of the Rollercam brake - a bit of nostalgia! This bike also had a 24" back wheel and 26" front as standard. It's a 1988 Cannondale - fully rigid in those days.
