I've been a proofer and playtester for all of these projects, and I think that The Road of Kings is by far the most useful of all of them.
If the entire Mongoose line of Conan books had stopped at this one sourcebook*, The Road of Kings would give me enough for years of solid campaign ideas and world background.
If I want to use another system other than d20 for a Conan campaign, that book is still invaluable.
If I have a friend who is a non-gamer but a Howard fan, this book would be an ideal gift for them as well.
* as a contributor and current author, I am overwhelmingly happy it hasn't