I have a Kindle Fire and bought it for the same day as print comics. Once Comixology works out their software bugs it will be great. Right now you have to go buy them using a real computer and then download them to the device. At first you could buy them right on the Kindle.
But reading comics is a great experience with Comixology - there's a mode where it zooms in on the current panel and just advances to the next panel with a tap. And I love being able to zoom in on the art.
However I HATE the android apps system. No where near as intuitive or functional as the Apple system. Once DC comics become available same day as print for ipad, I'm switching over.