I live and commute in areas where crime can be very high, and bike racks are hard to find. This lock is very spacious inside the U shape, with a little over 4" width and slightly over 8" length, which means I can lock the frame to many things not intended for bike locking such as street sign posts. If there is a bike rack, it is often possible to grab the frame, the rear wheel and the rack (far from the ground so leverage attacks will be harder to do). I combine this with a 4' cable to grab the front wheel, also.
Though every bike lock can be cut through eventually, the thickness of the steel and the fact that it would have to be cut twice in an unstable position and an exposed environment makes this pretty tough to beat compared to other bike locks.
I have been very happy with this, but it is very heavy-- even mounted to the frame and hanging down-- you will still feel the extra weight on most bikes. On the other hand, the only other lock that enables me to feel confident about leaving my bike outside in the city weighs 3x more. It also costs a lot, but the heavier locks are even more....
This lock is pretty hefty, however after having a cable lock cut on my beloved road bike I decided to invest. This lock it only for securing the bike in my bike garage, because I do not commute with my road bike. It is very heavy but I have peace of mind when I leave my bike in the garage at night now or if I need to run into a store. I also use the kryptonite cable to lock the front wheel through the main u-lock.
Look, $ is a lot for a bike lock. I've bought entire bikes for less. But $ buys you quite a nice setup, the mount is secure and doesn't slip around despite the heft of the lock, the lock is very strong, and the key opens the lock smoothly and easily.
This is a big, heavy lock, and you can feel the weight when you go up hills. It is also so big that when I carry it mounted beneath the top tube, I can't add any water bottles. However, I didn't buy the smaller version of the lock because I often find myself needing to lock my bicycle in an area where there's no bike parking, so I need room to get the lock around stationary objects not necessarily intended for locking.
So far I'm satisfied. No one seems temped to try to loose my bike after dark anymore. My biggest compliant is how massive the thing is as compared to how medium duty the bike attachment is. If you ride hard, the lock goes all over the place and after only two months of riding, the mount has come loose enough where I had to take the thing off and re-secure it. Comes with three keys which is nice if you're a loser or a misplacer as I am. The insurance policy seems substantial enough but I don't want to test it.
In short, if you really need security, this is THE lock. If you need compact or lightweight, this thing is a heavyweight.