New D800 owner opinions wanted

I would never consider "moving up" from DX as DX is much better than FX for some photographic tasks. For around 35% of my current work DX is the better format. If your work mix is similar consider keeping the D300

My current bodies are a D800 and, pending a D400, a D7000.

You need to budget for buying some good FX lenses for the D800 to get the best out of it.

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Leonard Shepherd
Many problems turn out to be a lack of intimate knowledge of complex modern camera equipment.