The PRS-505 is a very nice machine, but MobiPocket is THE dominant format which every make of eInk device EXCEPT Sony supports. There's a lot to be said for getting a device with MobiPocket support. I have the CyBook Gen3, which currently offers the best support for this format (and also, unlike the Sony, has a world-wide warranty).
Yes, one can certainly shift format, but why bother, when getting Mobi books is so easy? Lots of different retailers, and none of that "go away if you aren't in the USA" nonsense.