I did it in 2005. It is a paid internship. It probably could have opened doors for me had I applied to more NASA labs for full-time jobs when I graduated... however, it did offer a great conversation starter at interviews.
Also, the year I applied, it was very competitive -- I don't remember the exact numbers, but it was only at Goddard, and they only accepted like 30 out of several hundred applicants -- but I imagine it is similar this year now that word has gotten out.
I can't say it could ever be a *bad* thing to go build robots for NASA
