I just tried it on ds's trousers for school. Poor chap has a bit of 'girth' and short legs. I took up 5 inches with the iron, it is sad that the hem is not the same size as the higher up bit, so there is a big bump in one place. Not impressed.
If you're using the iron on stuff you can't generally let it down again as it marks the fabric. It's fine for a short term repair but I find that it is not sufficient if the hems are likely to get a lot of wear. If it's thin fabric you have to be very careful not to get creases in the fabric, it's quite hard to get it neat. With thicker fabric it's fine.
I find it unreliable for long term use. Hems come down after a few washes but it's good as a quick fix. My DSs have the opposite problem - long legs and skinny waist. Thank God someone had the sense to make kids clothes with those adjustable elastic bits like you get in maternity clothes!
The person who could come up with an equivalent for adjusting leg lengths would be onto a winner!