Another reason I enjoy a camera that does not have a moving mirror is a quiet shutter mechanism like is present with the Sony a57 and a77. I assume this will be true for the a99 as well. I was at a wedding the other night and the photographer was shooting away during the ceremony with his Nikon and his shutter noise was annoying even 75' away. When I shoot at a wedding or a formal indoor concert I can take pictures any time I want without disturbing the ceremony or performance. Before using a camera with a traditional moving mirror I had to wait until the music or singing was loud enough to not disturb the performance. I won't use a camera with that noisy mirror slap in these situations. My next purchase will be Nex 6. And maybe I'll make the jump to the FF SLT. Perhaps it won't be too long before even the Sony SLT cameras will be mirrorless too.
I think the Sony a99 camera looks very interesting indeed. I have used Sony SLT cameras for over two years and prefer the EVF except when doing studio work. A number of people have commented here that using an EVF in the daylight is a disadvantage. I have not found that to be true. Much of my shooting is of birds in flight around the Great Salt Lake. A Sony SLT camera with a EVF is wonderful for this situation. I am able to see my camera adjustments in the VF and know what is going to the sensor as the final image. I can see everything easily to compose the shot and adjust the exposure on the fly. Also I never have to look down at the LCD between shots and do not miss action.