I have the locking caps from Van's and I am very happy with them. I have yet to run in to nozzles that will not fit, but I know they are out there. I, too, have seen fuel leak out of the key slot if the tanks are filled right up to the brim. Spare key's are an obvious "must".
I had mine engraved by Bill's Custom Engraving (VAF advertiser) and they look fantastic.
Cross country flying is the reason we built this airplane, so I feel more comfortable with locking caps when the airplane is parked at an airport away from home.