I've been using Sculptris in teaching and personal work for about two years now. I came to it after running a couple of open-source prints on the Makerbot at my last job and feeling ready to print something of my own. Tools like Sketchup and Tinkercad don't feel intuitive to me- I like 3D modeling to feel less like a geometry lesson and more like playing with clay. Sculptris is perfect for me- it's free, the tools are basic, and it's much easier to get the kind of organic shapes I'm interested in.
That being said, Sculptris has a learning curve. Models usually need a fair amount of tweaking before they're in printable shape, and it's easy to get bogged down with a huge poly count and crash the program. I always use Tinkercad when teaching 3D modeling for printing, but I've had a lot of fun running Sculptris off one computer connected to a whiteboard so students can get a taste of a very different program. My favorite prompt is "make a smiley face"- with mirroring on, two strokes with any tool results in a satisfyingly weird result. Students usually take turns on a single model, generating freaky faces and impossible shapes in rainbow or chrome.
Some of my favorite resources: