Thus far it's finding bugs in OS WebDAV clients rather than the other way round. Nautilus for example has a rather nasty bug where it switches free space and used space statistics, which means that if you have an empty drive, it reports you have zero total capacity (!)
[note that DavFS2 does work Ok, the error is in the GFS/Neon layer)https://bugzilla.gnome.org/process_bug.cgi
Windows 7 client doesn't report usage at all if you use "Network Places", and if you use a drive mapping, it reports the free/used space from drive C: rather than the WebDAV share. (and it does the same if you connect to a WebDAV enabled Windows IIS server, so it ain't my server!)
Apple come out of it rather better, not only do they get the above right, but they do proper V2 stuff including locking and versioning, but then their Finder app seems to use the OS's WebDAV interface, which seems pretty stable. They also support PUT chunking which makes it feel a lot smoother when doing transfers.