Three days ago, I did a short piece about using my android tablet to read eBooks. Now there is a new version of FB Reader for Android, the very app I use for my Tab. It reads Mobi (Kindle) and ePub (Nook) formats perfectly. It's performed really well for me, and the new version simply fixes a couple of bugs that I didn't even notice.
If you're considering the Nook or the Kindle, I suggest getting an inexpensive Tablet instead. I did. With the FB Reader app, it does a great job of reading eBooks, plus many more things as well.