I don't think Maguras rim brakes can be called "V-brakes" as V-brakes are a subset of linear pull cable-actuated cantilever brakes. Maguras are hydraulic rim brakes. And yes, they're quite powerful as others have attested. I've even heard stories of them crushing some rims. As far as good cable-actuated cantilever-type brakes go, my vote goes to Avid. I'm running Avid ArchRival 50s and they provide plenty of stopping power. The SD series are also quite good.
BTW, anyone remember the Pederson SE (self-energising) cantilever brakes?