From my reading (mind you, I don’t have specific experience), the general consensus is that they’re a bit awkward to balance under the chin/jaw – requiring a bit more help with balance by the left hand thumb than normal. Other than the dimensions, I also understand that the body shape change allows for a much more sonorous low register, but also has the tendency to constrict the upper range. Most have some issues with their A string being a bit more shrill than normal. To me, if you’re interested in that rich low-end, it would be worth the time to find a good, warm A-string to match the instrument.
I’m not entirely sure of the specifics, but from the looks of it Michael has a viola that (from pictures) seems to be a Tertis-style instrument. The lower bout looks to be a decent amount larger.
I’m referencing the Sandro Luciano viola on Superior Violins. I’m yet to get my hands on one to confirm, but he may be able to answer your questions about it and maybe even allow you to try it out.