It's a crapshoot.
This update looks like a lot of trouble. Not the easy one two jump that lion was from snow leopard.
The way I remember it, jumping from SL to Lion was also a big headache for a lot of people (maybe bigger than ML upgrade). If you've seen the thread here (on the old board phpbb), it was several pages long.
Though personally, my upgrade from SL to Lion went very smooth without any hiccup. It was a non-event.
With this ML update, the only hiccup here is the AMD6870 card during installation process. But if you follow the simple step of deleting the kext, then restoring it back, you're back up and running very quickly on your next boot with full screen resolution/QE/CI etc.
I did encounter some bumps when some software/utilities I use were not ready for 10.8 or exhibited minor bugs. But after a day or two, 3rd-party developers quickly issued new version updates for their software.
10.8 is a BIG change/leap forward in the underlying "engine". It's not just cosmetic changes (in fact, cosmetic changes are few). Some of Apple's older OSX computer lineups can't/won't even run 10.8!
After a few days of checking all my software, I'm confident of my ML setup, that I overwrote my old Lion clone backup and replaced it with a new ML clone.
The only advice I can give you is make a clone BACKUP BACKUP BACKUP before you do anything.