A few people have asked me why, but except for one or two artists who found the comic panels and text balloons to be exciting additions, they haven't been sold on learning a new tool. And I can't blame them.
Here's a review by professional artist Brian Allen that I found interesting, and he explains a few cool things (like Reference Layers) that might help some artists decide to give this a try.
By the way, Brian has a collection of 200+ Manga Studio Brushes available on his site for only $7. I bought them and, depending on how useful I find them, I'll try to write up a review in the next few weeks.