Re: I've started monkeying with Terragen too
Looks good. The "spires in the first one are distinctive but have a bit too much contrast (its probably just the light)
I love the caldera in the second one, that's a very neat idea.
The third one's very good but it looks a little like it's been rendered on under full detail (on the top). Also the little spit in the bottom left looks slightly out of place.
Oh well, they're all very good and at least for you, your sculpt tool works, for me it doesn't seem to do anything.