I’ve had SpaceClaim since 2008. I had a small role in testing and giving feedback on the Rhino-SpaceClaim plug-in back in the early days of SpaceClaim. Back before the days of sub-object selection and the Gumball it was pretty useful. Now with V5 I rarely use it. It still has the edge over Rhino when it comes to editing fillets. However, be warned that unless your Rhino model is very simplistic, SpaceClaim will probably fail in it’s attempt to change the fillet sizes. I find about 50% failure rate when I send my model to SpaceClaim to edit my Rhino model. SpaceClaim does have some pretty good repair tools for fixing models but I find that if you send a model with naked edges to SpaceClaim, repair it, then send it back to Rhino, the model seldom stays watertight. It usually has different naked edges.
As a standalone software for conceptual design, SpaceClaim is awesome. As a tool to use as a companion to Rhino, I would pass on it. It’s not as useful as it once was.
Just my opinion of course…