Upgrade went through. Install scripts ran smoothly without any errors.
Install script disabled previous sshd config and created new one. Result: We were no longer able to login with our SSH keys. Easy to fix, but annoying. The install script shouldn't overwrite my custom configs.
The syslog daemon that is used to receive DMI device traffic logs in stream mode (SyslogOverUdp) did not start after first reboot. Had to manually restart DevSvr to make it start the syslog daemon. In the meantime, no traffic logs from SRXes could be received. No idea why it did that and I didn't feel like digging deeper. We'll see what happens when we reboot again.
Other than that, it looks good so far. It seems a little bit quicker overall (maybe because of the fresh boot though) and at first look, some annoying cosmetic bugs (like the search tooltip staying on screen multiple times) seem to be gone.
Kudos to Juniper for the interim bug fix release. Hoping to see 2012.1 (aka the big bug fix release) soon.