Android 4.x supports it, but I think the phone hardware has to also support it somehow. As far as I know there is no way to get a galaxy nexus or a nexus 7 into usb mass storage mode.
The data partition is ext4, so can't be mounted by most operating systems. It is also shared between apps, app data, and media storage so it can't be exported while in use.
IMO it's worth it to avoid that awful old chestnut where you'd run out of internal space with heaps left on the card, but unfortunately MTP isn't quite as convenient :(