Hey All... thought I'd share some thoughts and insights gleaned from my first outing with the a900 at a wedding.
To set the scene know that I was the second shooter... and due to the fact that I'm in the middle of a system change, I shot the whole day with the a900/sigma 50/f1.4. The main shooter, a good friend, agreed to let me use one of his backups as needed for wider/longer shots.
RE: The sigma.
Focus at distance with the 50/1.4 is difficult, to say the least. Inside about 10 feet it is very accurate and consistent. Outside (roughly) 10 feet, focus shifts pretty noticeably to the rear... so... long distance shots at large apertures are dicey. I'm keeping it, as it is sssoooooo nice and very accurate in tight, intimate situations (and the sharpness/bokeh are also fantastic), but might pick up the sony as well to see how it behaves at distance. BTW, choice of aperture seems to have little effect on focus shift on my copy.
RE: the a900.
Firstly... the color, resolution, DR (and viewfinder) all lends themselves to wedding work. The second cam I was using was a 40D and I had not realized just how bad the vf was until buying the a900. The camera is amazingly responsive IMO for such a high res camera... easily outperforming my older 5D in "snappiness". I'm quite happy with the files at iso800 and below. 1600 is fine as well, especially downsized to reasonable dimensions... but still quite noisy. I think as I get the courage to really push the histogram to the right, this will get better. But, man, I hate to take that chance with wedding dresses. DR seems to drop at higher ISO, but detail is still pretty strong. Chroma noise at iso 1000 and above is a bit too intrusive for my tastes, but again becomes less noticeable at smaller reproduction. The AF was accurate, for the most part... but I stayed in one-shot mode. Continuous AF (servo AF) seems useless on this cam ... but I'm hoping is better with the zeiss lenses. I'm not expecting it to be. I made the mistake of setting the AF to shutter-priority instead of AF-priority and found myself outrunning the AF in action sequences. In essence, I'd trip the shutter before AF lock and end up with OoF shots. Battery usage was actually pretty miserly. I have the grip installed and at the end of the day I exhausted one battery and had about 20% remaining in the second. This was after about 9 hours of shooting and roughly 2000 exposures... many using flash. Not bad at all. The explicit battery level readings are nice!
RE: the HVL-58AM flash.
In general, I really like the 58. Took a bit to figure out the swivel mechanism, lol... but it's growing on me. It's pretty powerful and recycles fairly quickly. TTL worked quite well and bounce flash tended to be fairly well exposed. It might just be me, but the pre-flash seems a bit more noticeable to me than on the canon flashes. Strangely, I did overheat the flash at one point... and it wasn't even a very active moment. I can only surmise that because I was at a lower ISO, that the flash was working harder than I realized... and being that it is silent as the capacitor recharges, I must have been overdoing it a bit without the requisite "whine" to remind me how it was working. After a brief cool-down (a minute or so)... it functioned perfectly the rest of the night (about 4 more hours) with no more overheating.
So... in the end i found the camera very capable if one is willing to shoot smartly once things got dark (ie lower iso with supplementary bounce flash). AF is accurate if not a tad bit slow (could be the sigma though). The camera is very responsive and the resolution, dr, color response, and general handling are well suited to most wedding work.
In all, a tough but rewarding day.
Enough talk... a few images: