As a guy who has spent his life working with houses I'll be the one naysayer to the idea. I see spray foam trickling into residential and commercial construction and I'm not a fan.
Yes, all of the things everyone says about it are true. It generally works better from a performance perspective. The downside is the inability to work with/around it later. You get another roof leak? First off you probably won't notice it until it's caused substantial damage since the foam will soak it up and potentially channel the water down the side walls (more damage). Then, when you go to make a repair it is a total PITA.
Maybe I'm just an old dog but well installed bat insulation and/or rigid foam will be 90% as good without the initial expense or potential headaches down the road.
