For the move tool, or for positioning the beginning of a line or end of a straight line with shift, the current snap behavior is fine.  But when using a drawing tool, after you start moving the cursor with the pen down, you get a nice trace of the guide or path until you get further then the snap distance.  Them 'blip' the line is drawn instantly from where you expect, to your cursor location.  Also if some other path or guide is closer to the stylus at some point the cursor will 'blip' over to it drawing a line between them.  Sometimes it will just jet away from the path several pixels and then back with no obvious reason why at all.

I don't think this is ever what you want the snap tool to do with the pen down.  I think this is why no one uses snap to trace paths, no one.

Suggested correction:  When using a DRAWING tool once the pen is down, make the next and previous pixel along the path or guide (possibly at a distance the current brush at it's current size is set to repeat at) the only pixels the current brush can go to.  Disallow snapping to other guides, paths or grids until the pen is lifted or mouse button released.

This would allow tracing a path or guide with paint tools, giving them nice variable line thicknesses and more character.  It would also make drawing while guides are on the canvas less of a pain, since unless the START of a line is too near, the cursor will not do anything unexpected like suddenly jumping to them.  Hence it will not be necessary to turn off snapping to draw some regular lines as often as it is now.

For folks using GIMP to ink their art this would be VERY cool.  In fact it would be so cool, if you do this you should add a new function to your accelerator table, jump to selected paint tool.  Draw a path, hit the accelerator and what ever paint tool was selected in the stroke path dialog, bang that would come up, and if necessary 'snap to path' turned on. When the user was done, hit the same accelerator and toggle back to the path tool.  Path-Tool perfect, variable width lines, with a work-flow smooth enough to allow complex images to receive this treatment.