Crescent Arch is the most obvious feature on the west face of Daff, fully living up to its name. As a friend of mine put it, it is a climb with an identity crisis - it can't figure out whether it's a face climb, a crack climb, a stem, layback, offwidth or chimney. Different climbers end up employing various combinations of these methods to deal with this tough, sustained, yet beautiful and well-protected line.
If you can handle it, it's a great alternative to the crowds lining up for West Crack, plus the belay ledges are much more comfy.
Use the same approach to the west face of Daff as for West Crack, but go left for about 100 yards until you are right below the arch. There you will see some blocky "fourth class" (actually more like 5.6-5.7) leading up to a belay ledge about 150 feet above the ground.
Climb the "fourth class" pitch to the ledge. From there, the arch portion begins. The next pitch, while technically not the crux, is the one most people find the most difficult. A bouldery start gets you into a sloped chimney where you must either grunt your way up, or else move out onto an unprotected face with a nasty fall potential and climb directly up to where the corner turns. From the corner, climb a sustained 5.9 crack-corner using a combination of liebacking, jams, stemming and smearing, until you reach a small but reasonably comfortable belay ledge. The next pitch is somewhat easier, with more features on the face for your feet, but still solid 5.9; the belay ledge that follows is the most awesome I've ever seen anywhere. The next pitch contains the technical crux (originally 5.9+, upgraded to 5.10b on Supertopo), but the climbing leading up to it is very well protected and the crux fairly short. With a 60-meter rope you can also pull the bulge out of the arch onto the face above and belay on one of those ledges; otherwise you have to make a hanging belay right after the crux move. The last pitch wanders up easy ledges with occasional 5th class sections to the top.
Make sure you have doubles on all the small stuff; this thing eats nuts like nothing else. Bring plenty of long slings for all the corners and roofs.