I have both and I wouldn't say I used both regularly, the nook color has sort of taken over as my dominant ereader (much to my surpise) but I do still use my regular nook. When my nook color is charging or it's all the way in the other room

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but that will change once the weather starts to warm up. I'm an avid outdoor reader and my nook eink will be my go to device for by the pool, out on my deck or any other outdoor reading.
I think they compliment each other perfect. Where one might have a shortcoming, the other fills that void.
They are like Peas and Carrots