Hacking down a SIM card to fit in a Micro SIM slot is a bit disconcerting - but this neat little punching tool worked very well indeed. It's made of metal throughout and feels reassuringly robust.
I ran a trial on an old SIM before hacking at the SIM that was my real goal and managed to cut both down to micro-sized cards without problems. To cut your SIM down, you insert it into a slot at the base of this punch. The SIM card slot is a good, but not tight fit and to cut a standard SIM down to Micro SIM size takes you VERY close to the edges of the data chip - so there is a bit of room for error caused by slippage of the card.
I found myself automatically using this upside down. Thanks to dips in the underside of the punch, it's not the most ergonomic way to do things... but this way you can see that the data chip and the cut out section on the punch remain lined up as you commit to cutting.