Earlier this week Readmill released an iPhone version of its ebook reader. Previously their app has been iPad only. In general I find the reading experience on an iPhone to be good but not great. However, I find myself reading quite a bit on the iPhone, as I can read basically at any time.
In general I find it ok to sacrifice the 'production quality' of media experience to be able to see, watch, play, listen or read it on a mobile. Most of the time some experience beat no experience.
Media experiences are a combination of product experience and content. Readmill has the former, but lack some of the later. Especially if you've been buying books for your Kindle, which uses a non-supported proprietary format. On the other hand, I should be able to figure out how to get books bought at Dito into the Readmill experience as they both support Adobe DRM.