One thing I can be counted on to do is to ignore most advice about safely upgrading my system. After a very quick survey of Google, I went ahead and upgraded to Mountain Lion on my circa 2009 iMac. Common wisdom is to wait until all plugins and DAWs have certified that they work and have had the bugs shaken out. Hey, who wants their wisdom to be "common"?
So after allocating an hour (and taking 3 hours), upgrade is complete. I ran into a few updates that had to go along with it. My virtual machine software (Parallels) had to be replaced with a newer version as did the drivers for my Canon printer. Easy, but time consuming - particularly parallels since I had to then update the virtual Windows XP version I run under it - just a reminder why I don't enjoy Windows quite so much!
So after all that what broke? Sadly there was one casualty. I use a now defunct utility called "VSTAU" to allow me to run Mac VSTs under logic. The middleware coverts them into an AU wrapper so they can be used in Logic which sadly does not support VSTs. One of my favorite freeware synths (Synth1) no longer works. There are rumors that they will put out an AU version eventually so I'm ok waiting.
The good news is that all of my "mainstream", AU plugins seem to work without issue. I am using the Korg M1, Arturia Mini and Modular, Aalto and many others without any problems. I'm sure I'll see many updates and patches following on but so far so good.
So, since I don't make a living with my iMac and don't make a living with my music software, no reason not to upgrade. If your circumstances differ...well, maybe wait awhile to see what shakes out.