Decent deal if it has 1 - 2 years of Applecare left and there are no defects. Same speed as the 15" MBP.
The speed gains are minimal over a MacBook - the dedicated video card is the main thing, which helps minimally overall, but more when you are using programs (primarily games) that can exploit 3-d acceleration.
Both CoreDuo models are limited to 2 Gb total RAM. Downside is whether the machine has any of the rev 1 bugs still.