I wanted a lock I'd feel comfortable leaving my bike with while I'm at work/in town and even just ONE of these works perfectly -- but if you want to feel extra safe you could use two or three of them on one bike (as I do in high risk areas).
I live in a fairly safe area, so these are just right, but I might recommend using a different form of lock alongside one/two of these if you are repetitively having to leave your bike in high risk areas.
It's very strong and simple/quick to use. There's no way somebody would stand around trying to cut it open, as they'd need some very special/noisy tools. Also, I sometimes worry about my bike not standing up in place, but the length/weight of the lock means you can securely tie it up without it falling over.
The locks aren't too heavy, I can keep them in my bag without problems.
It comes with six keys -- and technically you only need one for all three of them.
My only complaint would be that, if someone had the same bike locks/keys, they could take your bike too... but if they've got a bike of their own, they aren't going to take yours. But it IS one other reason to use a different style lock alongside this one.
Finally, it's extremely good value. You can't get any better for the price and I'm surprised they didn't cost more.
I'm very happy with the purchase and would recommend it to a friend.