The MOBI format is still the most popular for US-rights ebooks, because you have to have a MOBI version to sell on the Kindle. However, Amazon seems to be deliberately killing the format. They refused to let MobiPocket release a iPhone version of their reader for example and they refuse to let EInk devices support both MobiPocket DRM and other DRM (e.g. Adobe or eReader).
I think there is a 50% chance that the Kindle in the UK will support ePub (Amazon has a mobile ADE licence for the Kindle after all), which would be the final nail in the coffin for the MOBI format in Europe.
The real answer, though, is don't buy into this DRM nonsense. DRM might or might not effect piracy, but it definitely means that you can't read your ebooks on all the devices (existing and future) that you may want to use. The only protection consumers have is to strip the DRM, and use the DRM-free version on their own devices however they want. There are real differences between formats, but Calibre has demonstrated that format shifting is by and large an effective defence against ebook obsolescence. It can format shift MOBI to ePub and ePub to MOBI.
As others have said, the MOBI reader software is still better on balance than Adobe's reader software. So the ePub format may be superior, but the MOBI reader is still a good choice. I use FBReader for both formats, and it is a viable alternative (with its own flaws) once you are DRM-free.
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