In reply to d conacher:
Not used X4 on rock, only felt them in the shop but gossip from those that have used them is they're not a patch on Aliens/Totems.
Personally I'd rank Metolius Mastercams up there with Aliens and Totems (I have a few of all three makes) but many others seem to disagree with me on that one. I never got on especially well with zeros - on paper, as it were, they should be great but they just don't seem as nice to use.