I couldn't resist upgrading to Mountain Lion right away. It's gone quite smoothly.
I have multiple machines to update, so rather than download 4 gb multiple times, I used the free Lion Disk Maker to make a "disk" that I could use on each machine. WARNING: If you want to make a disk you have to do it after you download from the App Store, but before you install (because the install removes the files). You need an 8 gb device. I didn't have an 8gb USB thumb drive handy so I used an SD card from my camera. You can do the same process manually (as I did with last time with Lion), but the disk maker utility makes it easy.
It took something like an hour to run the update from the SD card on each of my iMac and MacBook Air. I also updated to Xcode 4.4 (free from the App Store) and installed JDK 7u5 (OS X is now one of Oracle's supported platforms for Java). The extra installs were probably the wrong thing to do in terms of isolating the source of problems, but it was nice to get a bunch of updates done at once.
Here are the issues I've run into so far:
The first issue was Gatekeeper stopping me from installing programs that didn't come from the App Store. I went to turn this off but I found that you can control click on programs and override Gatekeeper, so I left it turned on.
TIP: Update all your software to the latest versions before upgrading to Mountain Lion. My iMac was up to date, but my MacBook Air wasn't. After the upgrade Parallels and Dropbox had been moved to an Unsupported Applications folder. I just downloaded and installed the latest versions and I was fine, but it would have been easier to update first.
I had to reinstall Mercurial tools (as I did when I upgraded to Lion).
When I tried to run the Java Preferences (Applications/Utilities) it said I required Java 6 and offered to install. I already had Java 6 but I told it to go ahead. It now shows Java 6 32 bit, Java 6 64bit, and Java 7. I made Java 7 my default (by dragging it to the top of the list)
Eclipse seems to work fine, but JUnit Max quit working. It seemed to be running the tests, but the success/fail indicator stayed blank. This could have been an OS X issue or it could have been from switching to Java 7. Rather than try to debug it I just switched to Infinitest. It has quite similar functionality and it's open source.
The D compiler couldn't find GCC, which I solved by re-installing the Xcode Command Line Tools (Preferences > Download)
Parallels Coherence mode has some issues with window layering that are annoying but not fatal, hopefully they'll fix it soon.
So far I haven't really noticed much difference running Mountain Lion. Despite the "200+ new features" there's not a lot that's significant to me. That's fine, I still prefer to stay on the latest version, for security improvements if nothing else.
See also: Upgrading to Lion