I have removed the default helper path highlighting for paths with the Spiro effect, and made a couple improvements in the Node tool shortcuts:
* If a node is already cusp (diamond shaped), pressing Shift+C again on it will retract both its handles. As this works for any number of selected nodes, you can always retract all handles in all nodes by selecting all nodes and pressing Shift+C twice. * If a non-smooth node is next to a straight line segment, pressing Shift+S once makes it half-smooth: it now has one handle aligned with that line segment. Another press of Shift+S will expand the second handle as well turning it into a full smooth node. If a node is between two curve segments, Shift+S will expand both handles as before.
With these changes, and if you disable display of node handles (button on the Node tool bar) and the flashing of the mouseovered path (in Preferences), it is easy to edit Spiro paths without any distractions: Drag/transform nodes to reshape, use shortcuts to switch node types. The only thing that's slightly problematic is adding new nodes: without seeing the actual underlying Bezier path, you cannot do this by doubleclicking on it. You can, however, always select two nodes and press Ins to create a new node between them, then move it to where you want it to be.
Overall, I'm very pleased by how easy it was to integrate the Spiro math into Inkscape using the LPE mechanism, and by how flexible our Node tool turned out to support Spiro editing almost out of the box. Of course further improvements are possible, but even now Spiro editing is fully workable.
I have created about a dozen letters of a new font with this new toy, in a few hours of work, and they turned out incredibly nice and easy. I don't know how long it would take me to do the same with Beziers - most likely I would have quit, frustrated, after a couple letters. Beziers are really clumsy and inconvenient once you get used to Spiros :)