Should we use debhelper? Why? Any exceptions? Did you used it before (which package)?
Yes. Ubiquity, largely accepted, does the right thing. No. Yes (oolite, aspell-ro - unofficial).
Yes, I think Debhelper is so commonly used that most of the maintainers know how to handle it. It provides so many pros that I'm totally for it.
Yes. Works in a predictable way, can be used even if a package does not provide any build system. No. Yes, all of them.
Debhelper is well documented, behaves as expected, has responsive and responsible maintainer.
Yes; debhelper is the defacto standard of building packages, and that's for a good reason: It's well documented and tested, it's used very often and therefore designed to fit maintainers needs, and is a very good compromise between "making it easy for the maintainer" and "doesn't hide what it is doing".
Yes: Ubiquitious, recommended by the maint-guide.
Yes. Does the job extremely well. Excellent when no configure or make scripts are available upstream or when the build process is otherwise not standard. Overall a great tool when some flexibility is needed for package builds.