I use ring locks with a optional plug in chain, then back it up with a U-lock if I feel uncomfortable with the location/situation. So far I have not had a bike stolen while using the lock and chain. My favorite thing about them is that they are always on the bike, and the key is captive until you lock it. Prior to switching to the AXA Defenders I have pulled a variety of stunts: Rode to the movie theater 7 miles away with the lock, but no key. Ridden to the grocery store with the keys and no lock...etc.

Several of my bikes have the braze-on fittings for the ring locks, those that don't they make mounting kits.

Aaron