I like 75% into a wall, 50% into another monster, and 25% if nothing's there. Then it's more versitile. Also, on a wall crushing note, stone heart is no longer a very good transmuter artifact. I say change the heart and then reduce the cost to 3 (2 for wizards).
We made an expansion and it is awesome. Really, you should check it out, especially if you're looking for some extra challenge.
Download over at ddmod.weebly.com!