I have owned the OM-10 and OM-9. The OM-12 will be similar to the OM-10 with less bass. They are a very open sounding speaker with decent bass for their size. If you can pick them up for $150 or less, they are a nice speaker. Definitely different than you standard box speaker. They do need room from the wall to sound their best.
That being said, the OM-9 was a MUCH better speaker than the OM-10 and closer to the M5-si. If you can find a pair in nice shape in the $200-300 you would be much better off.