This seems to be one of the puzzle types I come back to regularly. I just like constructing them. People seem to generally like the type and it's hard to find any of them anywhere else. Last year there was a one on the World Puzzle Championships made by Zoltan Horvath. Another one will feature in this weekend's 24 Hour Puzzle Championhip made by Prasanna Seshadri. These puzzles both need all 12 pentominos to be used, which is something I tend to steer clear of. I think the highest I ever went to was 11. I'd probably could make puzzles of size 13x13 or 14x14, which use all pentominos. But then it still wouldn't be a rule that you'd have to.