Last year a colleague and I took a laptop and some borrowable material out of the library to a couple of places by student cafes to see what kind of interest we'd get. (We originally planned to call the service "Laptop Librarians", but some of our other colleagues have very dodgy minds, so we ended up calling it "Library on Location" instead.) We ran six trial sessions, then Christmas and various other projects intervened, but we eventually wrote up our Library on Location report (pdf, 166kB).
Short version: it was fun, feedback was positive, staffing is not always easy.
We felt it was definitely worth further investigation, so we're now running a second trial with a fixed time and place to see if having a regular service increases usage through familiarity. One of the things we're doing for that is getting our wonderfully cooperative colleagues to collect desk statistics back at our respective home branches to see how statistics "on location" at the same time compare.