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if you shoot 12bit raw, and even lossy compressed, you are giving up some percents of image quality (more if you expose it wrong). you lose some dynamic in shadows, about half stop of over exposure and underexposure headroom. is it worth it?
IMHO, yes. I've been studying the differences between the different NEF compression options with the D800, just as I've done with the D700 and D300 before it. To keep a long story short, the reasons people bring up for shooting 14-bit lossless (smoother tonal gradations, better highlight recovery, more shadow detail) are true in a mathematical sense. However, unless you consistently underexpose your images by 4 stops or more, or overexpose by 2 stops or more, there is no practical difference. Your skies are not smoother (even after extreme adjustments), your shadows aren't any less blocked up, and you don't magically get an extra stop or more of latitude.
However, my average 14-bit lossless NEF is around 45 MB. My average 12-bit lossy NEF is around 30 MB. I just bought four 16GB CF cards and a 64GB SD card as backup. The difference in capacity is 1400 images vs 2100 images. That difference carries forward to card transfer times, file backup times, disk storage requirements, etc. Yes, disk space is cheap, but it ain't free. A 50% increase in storage efficiency is nothing to sniff at, especially when there is virtually no trade-off in terms of image quality or processing power.
I can post sample images if anyone is really curious.