In reply to Stiffinger:
I've soloed up to about HVS/E1 barefoot on the grit and upto about VS/HVS in the lakes. For lakes climbing it's almost as good as shoes as you can feel holds better and polish is less of an issue- stuff like troutdale pinnacle is cool barefoot. I've soloed alot at stanage Barefoot and it's not too bad, although small slopey pockets don't feel secure and cracks can be agonising.
As the other poster said, you almost exclussively use your big toe, which is immensly strong compared to any of your fingers. If you were using holds that needed greater strength of toe, then your feet wouldnt stay on.