ML Perspective
Not to act like I know it all, but I thought sharing my experience here might help a bit.
I've upgraded for Lion to ML on both my 2008 Duo-Core iMac and my 2010 white MacBook - not fresh install, but upgrade. My impressions:
1. It's generally much more stable and faster than Lion; SL was the best but, ML is a pretty darn good incremental upgrade over Lion.
2. My boot-up times dropped dramatically when I booted up without being signed into iCloud.
3. Post-upgrade operating speed noticeably increased after clean my caches and repairing my permissions in ML.
4. Safari is a mess. Go with Firefox or Chrome.
5. If you're a Dropbox user, upgrade Dropbox before upgrading to ML. Ditto with your VM if you do that sort of thing (note that Oracle's VirtualBox does not have a version that will run well on ML, according to Oracle).
6. If you're a Dropbox user, signing out of iCloud will also improve performance. If you absolutely must use iCloud with Dropbox, sign out of iClound and sign back in to mitigate performance issues (but your boot-up time will continue to lag while signed into iCloud).
7. iMessage is very limited in who you can text with...really nothing special here yet.
8. I've had great luck with Dictation! Takes a bit to get the hang of it, but it's great once you do.
So, the upshot...am I glad I spent the $20 for the upgrade? Absolutely! While ML does not live up to the hype (and, really, does anything live up to the hype?), my experience has been that it's a great upgrade from Lion in terms of stability and speed. That alone makes it worth the price.
As always, your mileage may vary. But this is my two cents. Hope it helps!