I have never used the corner clamps, but have a reasonable selection of other Bessey clamps, they are all extremely well designed and the best clamps I have. I can't imagine the corner clamps will be anything other than superb.
For mitre corners, I will always use some form of tennon to hold the pieces together if they are big enough to warrant the use of a mitre clamp. I use either a dowel (cheap Record dowel jig of an auction site - they are really really useful) or a biscuit depending on the size of the mitre. To hold the mitre together I use the mitre couner glue which sticks instantly it's basically super glue with an activator. I use normal wood glue in the centre of the joint for ultimate strength and mitre glue around the edge (unless it's very small in which case it's just the mitre glue.