Ugly tiepography sighted.
In my defence, it's probably quite tricky to get pretty ties. To start with, the curves should have a variable thickness through the tie, so we're automatically dealing with filled polygons at the very least. In addition, there's a bunch of functionality hidden behind the ties (for instance, export to MIDI) which is rather better implemented. And this was the result of a relatively short hacking period.
On the other hand, it was probably harder than it should be; there is a relatively odd use of data structures (notes in gsharp are currently read-only, so any modification to one involves creating a near-copy) that threw me for a while. In addition, the system layout isn't ideal for incremental changes. Still, I'm relatively pleased with it, because we can probably take my patch forward.
On a different note, I verified today that sbcl builds under the latest release of clisp. The clisp developers broke the build with their previous release by sabotaging all attempts to use their pretty-printer; pleas to use sbcl compilation as a regression test before releasing have so far been ignored.