I wore the PI knickers on AIDS ride last year, my butt was very comfy.
I also have Terry, REI, and some Castelli. Castelli's nice but the others while fine for training, casual or commute, I would not wear them on a century.
Knee warmers are popular and great for most folks, I have to keep my legs warm all the time so might as well wear the knickers.
http://www.teamestrogen.com/products.asp?pID=17198
Speaking of keeping knees warm, anyone use the embrocations? I have read they are good
http://www.torelli.com/record/record.shtml