I used Z-Subsonic for quite a while, and it generally worked alright, but there were a few annoyances I had:
- It was never clear to me if it was possible to manually cache songs or just show what was in the cache and play it, so the caching feature was never very useful to me.
- It does not have a "random album" feature like the web player
- After you choose an album to play, the browse view goes back to the top level. This is extremely annoying if you have any folder hierarchy whatsoever.
- Has not been updated for quite a while - although it works with newer iPhones, iPads and iOS versions, it does not natively support them.
That said, it does manage to play music without crashing, which is more than I can say for some of the other iOS SubSonic apps.
In general I find iSub superior in every aspect -- in particular the cache is much better thought out.