In fact, SANS offers a training course specifically for the CISSP exam that can also be used to prepare for the GISP test. Since there is a lot more name recognition for the CISSP (as opposed to the GISP), why bother with the GISP?
The only time it's worthwhile to consider the GISP is if you can't qualify for a CISSP, which usually means a lack of experience. The CISSP requires four years of relevant work experience, which a lot of people may not have. Thus the GISP becomes a potential substitute, for a couple of years anyway.
Though as I've written many times in the past, certifications don't necessarily indicate competence.
This was first published in October 2008