I have tried several magazines with B&N and once you purchase them, they're yours and, as far as I can tell, remain in your account indefinitely.
Of the ones I have tried, they seem to be DRM-free, but many are in a proprietary EPUB format called "
Digital Replica Plus" (or on
MobileRead wiki). The text version of the articles and table of contents are stored in standard EPUB format and
should work on any reader that can read EPUB, but the file also contains additional assets which are high resolution scans of the pages (presumably the PDF that B&N gets from the publisher). This makes the EPUB file very large. I have also read that the text mode is OCR'd from the images, so may have errors, but I can't confirm this.
Only the nook devices and apps seem capable of using this format and only if you download them (not sideload them). There is no mechanism to download these from the web. Be aware that SOME magazines that B&N sells (notably ones from Time, Inc.) are only viewable on certain high res devices (HD, HD+, and Windows 8 apps), so pay attention to the device list on the magazine's subscription page (and/or take advantage of the 14 day trial). The most recent update to the Nook for Android app lists that it is now compatible with Time, Inc. magazines, but all of my Android devices still show them as not available for that device.
Hopefully this helps.
