These are strong, durable and very useful. They are much stronger than wire rivet hangers and, more importantly, are more likely to stay on the bolt once you move above it. Lastly, these give you another three inches of reach over a wire rivet hanger… and on some routes that three inches is essential. They are much more durable than wire rivet hangers, which often get worn down and have actually snapped on me under body weight conditions.
Even though they are much better to leave as pro, wire rivet hangers still have their place. For one thing, wire rivet hangers are much faster to place. If you are racing up a rivet ladder, you typically want to only use wire rivet hangers, then leave a keyhole rivet hanger on every third rivet or so. Also, sometimes wire rivet hangers are the only thing that will fit behind a rivet or machine head that is tight against the wall.
5/16" rivet hanger
The only way these could be better is if they had a keeper sling to make sure you don't drop them while cleaning. When cleaning these on a traverse, or when they get a little stuck, it can be easy to drop them. I don't know the perfect way to make a keeper sling but below is a photo of how I have made them for my own rivet hangers.
showing a keeper sling for a RP keyhole hanger.
Also, I find that the 5/16" size can get get stuck easily if there is not much space between the bolt (or rivet) head and the wall. If they feel stuck, be careful about cleaning them; don't wail on them with a hammer to free them. There is nothing worse than breaking a rivet in your zeal to clean one of these hangers.
They are useful for any big wall with hanger-less bolts, which is all but the most popular big wall climbs. They are absolutely essential on many obscure big wall climbs that might have missing hangers or long rivet ladders with no hangers. While they come in two sizes, 3/8" and 5/16", I generally only carry the 3/8" because it will work on the most different sizes (but is less secure on 1/4" bolts). For a typical wall, I would probably bring two 5/16" and five 3/8" unless it is a route like Wall of Early Morning Light on El Capitan, which has TONS of hangerless bolts. In that case you can't bring too many.