i have been useing these for the odd shower screen i have to fit,
supposedly they have a wax core that does the cooling, and contrary to what peter said they say to run them at full speed, which is what i do and have never had a problem, iv only ever bought one and it must have done 50+holes by now, just remember to start it slow with the edge ( at an angle) holding the drill tight so it doesnt skid about, once you are in about a mil or so, give it the beans and you will get a hole.
http://www.ebay.co.uk/itm/Marcrist-6mm-Dry-Diamond-Tile-Drill-Bit-/112478177916?hash=item1a3038ca7c:g:IHEAAOSwsW9YwwUn