The OnGuard Bulldog Mini U-Lock is definitely one of the best u-locks on the market. The lock is very light weight, compact, and has a fairly high security rating. As long as the cyclist takes care to lock up to something secure this lock will only be able to be broken with power-tools.
I definitely love how small and light this lock is; I am a bicycle messenger and every bit of space I can save in my bag helps me because I can fit more boxes and documents in there. I've used this lock every day for almost a year and it has served me very well; even with the small lock size I rarely have trouble finding something to lock up to, which some people complain about.
Anyone that is looking for a small lock to protect their bicycle should check this one out; it's inexpensive, light, small, and secure, which are the best features in a bicycle lock.