I have been using Apple products since 1983, starting with the venerable Apple IIE, then upgrading to a IIGS, and then to Macs. I have also been using mostly third party software on all of the machines I have owned. In all that time, and from what I can remember, this is the first OS that problematic at the beginning. And, unless I am mistaken, I believe it is the first OS for which an upgrade came out well ahead of the initial release. (Actually, when I saw that in August, I already started to have concerns).
I will say, though, that after insuring that all my software was upgraded for whatever new OS was coming out, the upgraded to the new OS has always gone smoothly for me. Of course, I do take care of my machines from a software perspective, and also use tools to make sure my hardware (specifically disk drives) is functioning fine. That goes a long way towards minimizing issues.
In actuality, except for a needed upgrade for Onyx and Tech Tool Pro (Microsoft released one for Office 2011 the other day), I would think an upgrade for mw would be smooth, especially given that I do not use many Apple products. In fact, about the only one I use occasionally is Disk Utility.