So what is Readium? It's not an ereader, and right now it's not even an app. But in the long run it's going to be an open source reading app that anyone will be able to integrate into their own app or ereader. They'll need to provide their own DRM (if desired), of course. So if you were hoping Readium will help you avoid the Adobe tax, sorry but it doesn't look like that will happen.
But on the upside, in the long run Readium is going to make things a lot easier for ebook developers. There won't be a need to test an ebook on a half dozen devices just to make sure there are no quirks in the design. Readium should also improve the reading experience of the bottom-of-the-rung ereaders; hardware developers will be able to use Readium rather than whatever cheap crap they're using now (you haven't see the crap I have). Readium also could potentially spur innovation. Assuming it's built right, software devs should be able to add experimental features without having to code the entire reading app.
The IDPF expects to release a feature-complete implementation of Readium, including an Android app, in mid-2012. If you like, you can try an early beta release of Readium right now. There is a proof of concept release available as a Chrome plugin. Chrome doesn't have many reading options, so this is good news.