I really like my iPad for many reasons, but probably would not buy it just for its musical abilities. In particular, the USB camera adapter could be tighter - it often slips off as I'm listening and I have to plug it back in. The real techies would know better, but I don't know how universal the USB is - there seems to be some voltage compatiblity problems, and not sure it would work with all DACs. Then there's always the threat that Apple will just pull the plug on allowing audio at all, in light of the fact that it's really supposed to be used for transferring photo files. Not saying they WOULD, just that with each iOS upgrade, the thought crosses my mind.
I'm thinking if you're looking for a device that is mainly to play music by outputing digital to USB, probably a GOOD netbook is a better option. If there is such a thing. I've read good things about the Lenovo Thinkpad X120e, but I have no direct experience with any netbook.. Looks like they're on sale, though!
http://shop.lenovo.com/SEUILibrary/controller/e/web/LenovoPortal/en_US/catalog.workflow:category.details?current-catalog-id=12F0696583E04D86B9B79B0FEC01C087¤t-category-id=C63BB22720313B14F5906FFA72CB90D5&action=init