Now, I used to sketch stuff in 3Dcoat and I was using its automatic quad retopo, which gets rid of most of the details in my sculpt. Still, it can produce some interesting geometry really fast. In Sculptris, I just save the model, poly reduce it (in Sculptris or in Maya), and I go to Maya and quadrangulate, as much as it manages to... The results are not exactly very clean, but renderable, as is, or with subdivs at rendertime, depending on how dense the mesh is, and the process keeps the model's features pretty well, even with heavy poly reduction (and you can go pretty low poly). The result is also quite deformable too, if you only need to do simple, cartoony stuff. For cleaner results I'd need to do retopo by hand. But I don't need the clean results, I actually like this sketchy quality of the outcome :) And if I'd spend a day doing retopo on a model that took me 10-30 minutes to model... it wouldn't be exactly sketching. Well, this 3D sketching anyway is still nowhere near the speed of actual 2D sketching, but... it's pretty damn fast, compared to more traditional methods. And a lot of fun!
Edit - I just realized, one day later, that I can actually use Sculptris to recalculate the triangles of a dense mesh exported from 3Dcoat, and so now I can apply the same workflow to a 3Dcoat sculpt, yeeeeey :) (3Dcoat voxel sculptures exported as full rez, dense meshes, don't poly reduce so well, usually, but Sculptris recalculates their triangles and makes them 'reduce-able').
Re-Edit - Did I say scratch 3Dcoat?? Silly. :} In fact 3Dcoat has not only the best sculpting and retopo tools possible, it has an unbelievably fast, reliable, and smart poly reduction during export... I use that a lot. I export ALL kinds of stuff from Maya to 3Dcoat, voxelize the geometry, mess with voxels, export back to Maya as optimized models, as high or as low poly as I need them. Porky might have been modeled in Sculptris, but almost everything I do for my own puppetzoid junkytronics... I use Maya & 3Dcoat.