Thus far Mountain Lion seems stable, clean, has some noticeable speed boosts over Lion (even my wife commented on the improvement in Fast User Switching speed), and seems to have fixed a couple of things that the Lion Finder broke with handling links to network volumes.

It also FINALLY removes the boneheaded cross-browser delete-key shortcut for the back button from Safari, which has been a longstanding booby-trap that any UI designer could tell you was insanity from the start (it does something completely different depending on whether the focus is in a text box on a web page or anywhere else on the page, so an accidental tab followed by a delete can--and often does for me--result in you going back a page and losing the contents of a form, rather than just deleting a character from the next field).

On the down side, after forking out for Mountain Lion Server, I've discovered the harsh truth that the Time Machine server only allows you to serve a single partition, and doesn't allow for any kind of per-machine quota, which makes it nearly unusable for any situation in which you have more than one machine backing up unless you have vast amounts of backup space available. Something that, ironically, was never a problem with the non-server Lion (or 10.6, or 10.5), where you could share multiple partitions and just select one per machine to back up to, thereby instituting a crude sort of quota control.

There are apparently a couple of hacks to get around this, but since 10.6 Server had the functionality in place, it's sort of unforgivable that they broke it, especially since it's an obvious use case in either a household with several Macs or a medium sized office. I was planning on instituting TM backups across all machines at the office here, but it means we'll either be stuck on 10.6 Server or I'll have to hack quotas onto 10.8.

Really, guys, it's not an unreasonable feature to ask for.