Radiant barriers are great in theory, but in practice they usually fall short. The primary reasons are:
1. Once they get dusty, they don't work.
2. People press them right up to the surface they are trying to insulate (and air cavity between the radiant barrier and the next closest surface is necessary - at least 3/4").
Adding a radiant barrier to an existing form of insulation can't hurt, but as a stand alone product I usually find their effectiveness lessens substantially over time. So to answer your question: Unless you have a specific need where radiant barriers can perform, I wouldn't spend a lot of effort or cost seeking one out.