With the Zaurus DevNet going down, don't expect the tKC ROM to be around for much longer (at least not updates -- I think the ZUG download section has them mirrored). It also hasn't been updated in almost a year now... :-/
The tKC ROM is nice, but as I said, little work is being done on it, as far as I know.
The Zynergy ROM is nice. It's got an annoying bug in the IPK installer, unfortunately. Other than that, it's pretty great (of course you should probably use a different kernel, for the cache bug being turned off). Unfortunately it doesn't seem to be in development either... Correct me if I'm wrong.
The plain Sharp ROM is stable, works well, but is aesthetically unpleasant, and the default kernel has the cache bug workaround on (effectively cutting the CPU speed in half). Unfortunately with Sharp basically dropping US support for the Zaurus, don't expect any updates...
And then there's my Watapon (recently updated to 1.4, with no new updates planned until late December), which is stable (the cache bug workaround is off in it's kernel, but it doesn't cause instability for me), and aesthetically pleasing, at least to me. There's some screen shots here:
http://zaurus.duelmonsters.ca/screenshots/ I haven't noticed a single problem with it, anywhere. Also got a small IPK feed, which I will add applications to upon request. Of course you can take this entire paragraph with a grain of salt, if you wish, since it is my project, so I can't be
totally impartial.
All of the above are based on the Sharp ROM.
As I said about OpenZaurus above... OpenZaurus, while very nice, is too unstable for real use on the 5600 currently.