I used this lock to take my mountain bike all over the place, and I have evidence that it does the job.
My bag got stolen, which had my bike locks keys in it. Due to this, my bike was locked up in a dodgy part of town, and I had no way of retrieving my bike other than cutting it lose.
Tools we used: Fireman bolt cutters, handsaw, a van, a saw.
All of those failed. The bolt cutters left about an impressive mark on it as a toddler would if it chewed it. The handsaw did nothing, the van did nothing, the saw did nothing.
In the end we had to use a mains powered angle grinder and a diamond level saw disk, even then it took a good 15 minutes and the diamond level saw disk was in complete tatters, and had to be thrown away.
Not bad for £20. Although the difficulty was with the MAIN LOCK. The extra CABLE came off pretty easily.