I upgraded just for the dark mode which was 100% worth it to me.
Not sure if it has anything to do with Mojave, but a few days after upgrading, my MBP bricked itself when I opened it while plugged into an external monitor.
I did it accidentally. The dark mode is nice, but I haven't noticed any other differences.
Not yet. Some software I use are not upgraded yet. And I have still to find how to sign gdb to make it work on Mojave.