There's a plethora of cheap Arduino copies on eBay and similar sites, usually originating from the far east, that can be had for a significant chunk less than the official boards, especially if you're buying more than one.
Aside from the moral reasons of supporting the Arduino guys (not that I'm ignoring that, just not considering it for the purposes of this question) is there any technical difference between them, or are they pretty much the same (Arduino are open source hardware after all) will they be, as far as functionally, pretty identical?