I have 4 pairs too. I love them but the one time I walked a few miles in them (from school to apt.) I ended up with blisters (probably because it was the first time I wore them) but other than that I've never had problems. I esp. love my gold and silver ones.
they are made with a special rubber--the recipe for it is a heavily guarded secret at the manufacturer in brazil. they are cushier and more comfortable than other flip-flops because of the special rubber on the footbed. and also the actual thong part is softer--they are the only flip-flops that don't give me a blister between my toes.
The rubber, most defintely. It's really cushy so these are the only non-platform flips I can walk in all day. Plus, they come in tons and tons of colors. They also have a long shelf life, at least for flips. I used to wear the Old Navy ones and while they're great in their own right, I would have to throw them out after maybe 3-4 months of wear. I think I will be able to get at least a couple years out of the Havaianas.