[BrandonMTurner]

I used to work at Microsoft, and there was a few of these type of people you described. My personal feeling is that their up sides usually even out to their downsides. However, there is a chance it really is a good idea, you really can execute on the product vision, and he really can sell it.

The way I think you find out if he is valuable is not is simple (ha, I say this, but I have no experience actually trying this so I could be horribly wrong):

Tell him you will dedicate to X amount of hours to development over Y amounts of weekends to make a prototype. If he can take that prototype do something with it (investment, sell it as is or with little more work required, etc...) then you will join him as a real cofounder. I would suggest something in the range of 30 hours over 4 weekends but it may depend a lot on the idea.