I have two, a mini and regular sized u-lock, use one or the other depending on where I'm parking my bike. Never had a problem. Sometimes though, I have to rattle the key a bit to get it to work, it isn't exactly an easy lock (on either of them), but they aren't freezing up at all.

+1 for crappy bike mounts though, just Tuesday my mini came flying off the mount while I was going down the road. The other one, though, once it is locked into the mount, it ain't coming off without a lot of hassle.