Stability has to be second to security - and releases prior to 9560 have security issues that have been patched/fixed - some of them severe.

So far, all of the recent versions have shown to be relatively problem free, judging from observing the forum traffic and conversations.

The best solution is to run the most current version, and if there are issues, open a support ticket and get them addressed.  If a support ticket can't seem to solve it - come to the forums - maybe it's something the rest of us can look for, test, and help recreate so that it can be fixed. 

SmarterTools can't fix things they don't know are broken - and the only way they find broke stuff is if we run it and report it. It sucks to be the guinea pigs with our businesses but it is what it is.