In reply to Rob Greenwood - UKClimbing:
Having used this type and the Grivel Candela, I just about prefer the candela. Both have their downsides: I've also broken a Candela, but in transit, not use, so more careful about storing it. I've never struggled with the candela in its actual use, but I tend to first thread through one of the really skinny 6mm dynema slings which always works well. My main reason for liking the candela, is the use I put it to much more than actual v-threading, de-coring screws quickly with it still attached to my harness.
The main reason I didn't like the OP one, was the hook cover always seemed to pull up leaving the hook to snag on trousers. In fairness, an annoyance of the candela is that after a while the retention clip for its hook seems to loosen and let it fall down, but I've never had it snag on trousers.
Maybe there's room for a new product that combines the best of both, with none of the foibles.