Having used ultimates on my trials bike for about 1.5 years I would highly recommend them. Very powerfull with a solid feel and of course, very light. The only v-brake caliper I think I'd ever use. It's just a pity they have stopped making them. Best snap one up quickly if you do decide that's what you want.
I also recommend getting a ultimate lever to compliment. Definatly better than an XTR lever. Very smooth, and very comfy. The only downfall is they seem to die way before the calipers. The levers develop play, I'm assuming due to the cartridge bearings, but then again all other levers have play in the first place. The levers also have a tendency to bend with relative ease. This may have been to my oafishness, but my of the 2 pairs I owned, they lasted between 6-9 months. XTRs definatly seem stronger. They just feel a bit crap.
Finally, if you want the perfect brake setup, get a ceramic rim and avid green pads. Words don't convey how amazing I think this setup works. Predictable modulation and loads of power with very little noise. Also works well in the wet :(