I borrowed a A900 over the weekend and shot it side by side with the 5DII and sometimes the D3. The only lens available was the Sony 50/1,4, so no Zeiss glass unfortunately. I also used the Canon 50/1,4 and Nikon's new 50/1,4.
The most surprising find is the difference in colour rendition. Even when developed in the same RAW converter (C1), although generally I find Canon and Nikon files look best in their native developers. I didn't try the Sony converter.
A900 files look notably better converted in C1 than in ACR IMO.
This brings up a question: has anyone noticed a lack of highlight warning for A900 files when converting in C1? I was fooled by this at the outset and didn't apply the highlight recovery function or pull the exposure to avoid blown highlights. They were clearly indicated in ACR and also the identical 5D2 exposures were duly filled in with red in C1, but not the A900 files unless there was really extreme overexposure.
I still have a bunch of files to evaluate, but initial impressions are good. I've only had time to print up to A3+ so far, but even at that size the resolution of the printer is the limiting factor as far as resolution of fine details is concerned. An interesting observation is that the difference in colour reproduction between the Sony and Canon is at least as visible in print as on the monitor.
For the seasoned A900 users on this board, the following comparision shots will probably be of little interest, but for those of us on the fence: these were taken within seconds of each other with the 5D2 and the A900, both developed in C1 with default settings. WB: daylight. The A900 file has had the contrast boosted a little to bring it to roughly the same level as the Canon.