I also use Keen sandals but also carry a pair of socks for those cold mornings. The Keens are great for commuting but I don''t like them on long weekend rides. For those days I use my road bike with Look Keo's.

Keen has two types of sandals out now. One with bungee cords and one with straps. the straps are stiffer but more expensive. I have the bungee cord style. These work great on my single speed commuter.