I don't have experience with that specific device, but I recently built up a Ubuntu based instrument using a bunch of USB boards for A/D, D/A, DIO, and temperature IO and relays. All the devices were marketed for the Windows world, but either had linux drivers downloadble or pointers to drivers someone else had made for their hardware for linux, and it all worked pretty well (most of it was from measurement computing). I did a kind of kludgy thing for debugging speed (since I didn't need high speed IO, and was on a really tight schedule) and compiled little atomic chunks of the C code that did the basic functions, then wrote perl modules to call them.
The board you're looking at is really just a multi-function IO board-- it's got 2 A/D channels, 2 D/A, and a bunch of digital IO-- there are lots of boards that will do this. What looks nice about that one from Digilent is price/performance, especially if you can get the student deal ($99!), for a 100 Mhz board. But it won't really be an oscilloscope out of the box. The software is really what gives it the scope features.
If what you want is a budget oscilloscope for dealing with other projects, it looks like there are a fair number out there in the ~$300 range, and you can get new or used ones on ebay. If you want an inexpensive high performance multifunction board that you can use as a scope if you wrap some of your own software around it, it looks like a pretty good deal, and I would guess that the library does work out fine.