I have an attic that gets direct sun until the afternoon. It gets quite hot. I had easy access to the rafters so I used radiant barrier, and the difference is very big. As you're putting it up you can tell that it's blocking the heat standing in an a covered vs uncovered area. In subsequent days when it was all up it was obviously cooler. It's still hot but not unbearable.

Radiant barrier is more expensive and fiberglass probably would have worked just as well in this situation, but I didn't know enough about air flow in that space to tell whether fiberglass would impede anything,so I used radiant barrier and left a gap at the bottoms and tops. It is very easy to install. Fiberglass wouldn't be too hard either, but the barrier is daed simple and there's less volume to move around.

In general, my experience say it's going to help, and whether you do fiberglass or radiant barrier is up to you.