Expensive??? Don't let me laugh! You'll find one second hand for €450.
My opinions are generally: A great synth, with very usefull sounds. A nice user-interface (the big display draws every waveform/parameter you're changing). It has also usefull effects: Chorus, Delay, Reverb and Overdrive with much editing options (well, the last one, overdrive isn't very usefull. It gives a shell, harsh sound, not like most guitar-overdrives).
I'm a big fan of the filters, because of their big resonance. That makes them suitable for the most strange fx and screaming leads

. They can be connected to each other in 3 different ways.
It is 4 parts multitimbral. On basicscreen you see the numbers 1,2,3,4, and there are buttons to switch between them. So you can switch between parts (to change parameters for example) while playing. Only bad point is that the Arpegiator/phrase recorder turns off when you switch part.
The arpegiator is just a basic arpegiator with 4 patterns, nothing special. But the phrase recorder is nice! It's a basic loopsequencer, but with much editing options. You can store 4 different phrases at a time. (thanks to a bug in the OS it became 5 by my OB12

)
But there are ofcourse also things that are not good: The OB12 has a bit limited polyphony. not too less, but 12 voices for 4 parts isn't always enough. Second: it has a bit latency. Especially if you use much of the voices it can become horrible. Further, even the last OS has some bugs. And those bugs always come when you don't want them. Mostly storing a patch is enough to stop the bug, but sometimes you have to restart the synth.
I'll try to make a video-demo once, but I don't have a camera...