Mini U-locks make sense; a smaller shackle makes them very difficult to lever, or indeed to get any sort of levering tool inside the shackle. The downside is that it limits the applications, and the convenience of the smaller size is offset by the need to use a secondary cable (although that's something we recommend anyway). The Fahgettaboudit Mini is the stiffest, toughest lock we've ever put to the sword. In test one we couldn't make it budge at all - no amount of hammer, axe or lever even made a dent. At one point we used our 4.5ft lever in the shackle and two people (25st+) hung off the bar and it still didn't budge.
Unlike the excellent Abus, in test two this one only required the one cut, but the material used is very hard. It resisted our cutting tool (destroying a cutting disc in the process) and it took close to three minutes to sever it. It's expensive, too - the size is limiting, but the price is offset by the $4,500/£2,450 guarantee (the highest we've seen on any lock) and if you supplement this with, say, a Kryptoflex cable (£4.95 for 2.5ft) the size is less of an issue.