I use the original Droid and for me it is a backup for my Kindle. Not everyone has eyestrain with LCD screens but I do so I do most of reading on my Kindle. Plus, there is a Kindle app that allows me to read the same Amazon purchased book on both my Kindle and Droid and keep the book synced between the two devices. That way I can leave my Kindle at home and read on my Droid when away for a few hours. If I'm going to be gone for the day or longer then I will bring my Kindle because for me it is a far more comfortable device to read on.
I haven't tried to send books wirelessly to my Droid with Calibre so I can't help you there. But it is easy, peasy to send books to your Kindle wirelessly with Calibre. And if you use the wifi, it's free.
Personally, I find the 4.7" screen adequately sized for reading but I strongly dislike reading for a long time on a LCD. The Kindle app and Aldiko app can change the background and font colors and I generally use the night setting (white letters on black background) because it bothers my eyes the least.
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