In reply to gingerkate:
Have used sealskinz on a 50 mile running event. No blisters. Really comfy, didn't let my feet get too hot. Stayed waterproof for 42 miles - nothing was going to keep the horrendous weather out on that event! Even then, feet still stayed fine temperature-wise.
Word of warning - be careful how you wash/dry them. They have an inner lining which became detached in mine and would runch up and so became unusable. I sent them back, Sealskinz accused me of washing them too hot (over 40 deg) or drying them on a direct heat source (eg radiator) - neither of which I did - my washing machine is programmed at 30 deg. Needless to say I got no refund or replacement product - not impressed with their customer service.