Southpaw, they are incredibly easy to use. I have only used normal plaster thusfar, but I want to pick up some Merlin's Magic as I have heard great things about it. Anyways, no single cast will get you far, maybe a small ruin, but around 10 or so can build you something nice. Normally to get a good sized building you are looking at 30-ish. Ruins take a lot less and are far more forgiving for mistakes, also much easier to game on.
I have built a few structures straight up, including my dungeon and the Warlock's House (on his website, also my first build, came out pretty bad actually), and use them to enhance, both work great. The molds I have are really nice for fantasy, and work best for areas where worked brick would not be common. Heck, they'd even do well to make 1" high fieldstone walls to fence in sheep and such.
Hirst Arts has some amazing stuff, and I have been more than happy with my purchases, but I want to move on to other molds more fitting of the stuff I use, so worked brick and maybe the Egyptian (Gotta cover my Elves and my Stygian)
Just go over to his website, he has a ton of great data on his molds and how to use them.
If you want mine, I'll give you a nice discount

Later,
Glenn