I bought this lock to protect my Specialized Hardrock Sport in the bike theft happy region of greater Chicagoland. Checking my local journal (The Wednesday Journal of Oak Park, IL) will show bikes get stolen near me at the rate of 3-5 a week. Having a lock, and a solid one at that, is pretty much a must.
I chose this lock because it was recommended to my by my bike mechanic. It is a very sturdy, heavy duty lock that screams "look elsewhere" at any potential crook. Since most bike thieves steal based on convenience, chances are good that this lock is sufficient to deter any potential thief, unless you're locking up some 10k custom bike.
PROs:
1) Very sturdy design
2) Comes with 4 keys, meaning you can have a couple on key chains and a few more in reserve 'just in case'
3) Easy to open and close
4) Comes with an anti-theft guarantee for up to 10k for a un-powered bike
Cons:
1) The bike mount attachment (to connect the lock to the bike) is majorly flawed. It relies on a plastic inset to tighten a nylon band around one of the poles of your bike. It does not tighten sufficiently, and once attached, cannot be removed without damaging the plastic inset. It's a terrible design! Additionally, the lock clip which attaches the lock to to the mount sticks all the time, making taking the lock on and off a real hassle. Sad that they combined such a great lock with such a crappy attachment. I just carry the lock in my backpack, which isn't ideal, but works.
2) The lock is very heavy. That's to be expected since it's beefy, but if you're worried about weight, this is probably among the heaviest U-locks you can find.
Conclusions:
This is a solid lock that will keep your bike safe. The mounting attachment is very poor however. I'd still recommend it.