I don't need a lock for commuting...I just wheel my bike into my lab.
But, I'd like one for running occasional errands to the postoffice, grocery story, book store etc. I wouldn't plan on locking it up for more than a few hours at a time, and I don't live in a very sketchy neighborhood. I have three bikes (Redline Monocog, a Specialized Sirrus with drop bar conversion, and an old Bianchi fixed gear beater). None of these are highly valuable, though I would just die if any of them was stolen. I'm not in love with the idea of bolting a lockholder to my frame, nor am I in love with the idea of lugging a huge chain around my waist.
Any suggestions, given my needs. I suppose I want something with reasonable security but something lighter than a giant chunck of lead that I could just toss into my backpack. Whats a good compromise?