Jen,
I have the GDRI, II & III plus the GX200.
The GRDIII has only been with me a few weeks, so I'm still using it in all sorts of situations to see how it performs, but it really is quite a significant step up from both earlier GRD's.
I now find I'm using all ISO speeds, and colour accuracy is very good. I agree with Mitch too, that DR is much improved. Add to that the range of Snap modes now available and this little cam suddenly becomes highly versatile in a way the earlier models never were. But its also a very easy cam to learn to use.
In short, it well extend by some way, the range of subjects that its possible to use this camera on confidently. I was out and about this evening in full darkness at a busy fishing port, and was happily snapping away indors and outdoors all evening.
I'm very pleased with the results, and my GRD II will be sold, and probably the GRDI as well.
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Regards,
Rich Simpson