I recently acquired a Sony A99. I have been pinning for this camera for some time (the EVF, Sony sensor and fully articulated screen were very appealing to me) but balked at the initial asking price of almost $3K. I was finally able to get one used but in very good condition for under $2K. For the foreseeable future, I will keep a dual system of Canon 6D and A99 and maintain a rolling comparison of the 2 cameras on this blog.
My first impressions of the camera after having played with it for a few hours shooting on a Tamron 28-75 2.8 (which I now have a Canon and Sony version):
1. This A99 is quite light for a full frame; it's almost as light as the 6d and the difference in weight is hardly noticeable
2. Ergonomics on the A99 are wonderful. I have always had preference for Canon's beefy grips, well Sony's is slightly bigger and feels slightly better in the hand. A99 has 2 conveniently located scroll wheels, one for your index finger and the other for your thumb. The 6D has a scroll dial on the back of the camera and scroll wheel for your index finger. I prefer Sony's set up.
3. After a few test shots; I can quickly say that the A99's metering and auto white balance is significantly more accurate than the 6D
4. Looking at unedited raw files, ISO performance is about the same from both cameras
5. I am surprised at how clustered in the center the A99's AF points are; The 6D's miserly 11 points cover a slightly greater surface area than the A99's 19 points. As I understand it, I can get one the 17 lenses compatible with the A99 phase detect AF mechanism thus increasing the AF coverage area and the number of AF points to 121
6. The Tamron lens is a lot noisier (actually super loud) on the A99 and also hunts more than on the 6D. While I really like using this lens on my 6D because of its light weight and stellar IQ; I can already tell that I will despise using it on the A99 because of the loud motor. I'm surprised by this, because I don't imagine that Tamron used different types of AF motors. I suspect that this is an aberration especially considering that I got the Sony variant of the lens through an Amazon Warehouse deal
7. Battery life is incomparable between the two; the A99 drains its battery very rapidly even with the GPS turned off. You will probably need 2 batteries for a full day of shooting on the A99. The 6D will carry you a few days on a full battery.