The Black Diamond Vaporlock is an awesome locking carabiner for belaying and rappelling. It is light, provides good friction, and has a keylock gate. It is ideal for belaying and rappelling where you want a little extra friction. The flip side is that this is not the best biner for belaying off the anchor. It is one of the lightest "full function" locking carabiners out there. It's main competition is the Petzl Attache which is just about the same weight (just a few grams more). In general, we like the Attache 3D more because you can belay off the anchor more easily with devices like the Petzl Reverso, it is better for use with pulleys like the Petzl Mini Traxion, we like the gate more, and we just like the feel of it more because it has an I-Beam construction. That said, the VaporLock is $5 less expensive, adds more friction when belaying and lowering, and is also one of our favorite carabiners.
Overall, this is more or less tied with the Petzl Attache 3D as the lightest full-function locking carabiner. If you just want a good value for a locking carabiner, we would go with the Black Diamond Rocklock. If you want the lightest locking carabiner, we would get the Wild Country Neon. But if you are looking for a really light, full function carabiner, that is moderately priced, this is hard to beat.