In reply to Brendan:
I got the Rockfax as a lot of friends had ground up and the coverage is slightly different (RF includes the upper tier - low grade single pitch - bit of tremadog, GU seems to have better coverage of Pen Trwyn).
However, the RF guide may have better slate coverage than GU, but the map/descriptions of approach are somewhat lacking.
Specifically the maps lack key features and the approach descriptions are across several pages, e.g. Serengeti area approach is mostly via Watford Gap, however Twll Mawr (in the Serengeti area) is approached via a turning and ledges from Dali's hole, cue turn around, retrace steps for 10 min.
So if I had the choice again I'd go for GU, or individual guides. I'm not bitter about one failure to read the approach description, honest.