I've been using an mbox2 as a external sound card on windows- it sounds better than my onboard soundcard or my ipod, and pretty much rocks my socks. There's a few small hitches - it only works with apps that supports ASIO, and only supports a single stream at a time - which is not too bad- system sounds work over the normal soundcard, and music over the mbox2.
Now.. there's a experimental driver for linux which, while incomplete and unsupported, and possibly not going to be updated in future, that'll let me use the mbox2 in linux, and dosen't have some of the shortcomings of the windows driver. However this means compiling a whole new kernel apparently, and i'd rather not do that. I'm wondering if there's a way to compile this as a module (the comments hints at this) on its own, rather compiling a whole kernel.