Speaking as someone who's never touched splines before, this looks pretty nice. I was able to very quickly reproduce the "a" from your screenshot using your demo.
Thanks, that's music to my ears! A major focus of this work was making a primitive that would be intuitive and that you could build a good UX around. Considering the nontrivial learning curve for Béziers, I think your report is significant.
Also as someone who's never touched splines before, I found it pretty difficult to replicate. The "a" from the screenshot looks fairly smooth everywhere (except possibly the upper-right outside curve), but I found that unless I got the positioning just right, I could run into some degree of "bumpiness". This may, of course, say more about me than about Raph's new splines :P.
If I wanted to try Raph's prior work on spiro splines for comparison, is there some way to do that that's easier than learning to use Fontforge? (When I last tried Fontforge, it didn't seem to handle hi-DPI screens well...)
