I deliberated for years over adding rivnuts for a water bottle cage to my Raleigh Gran Sport. The bike is no showboat, but I'm unaccustomed to drilling holes in 531 framesets. Well, I finally convinced myself it was the right thing to do, and after a couple test runs on a scrap piece of tubing, it was cake. The installation tool for rivnuts is costly for the occasional user, but I found the following method works very well. I ordered the rivnuts from McMaster Carr, pt. #95105A175.
Clearly there are frames I wouldn't drill, but for all the rest, this provides a nice and pretty simple alternative to clamps, handlebar cages or going thirsty, and does it with minimal damage to paint. I've read the horrors about rivnuts coming loose but my understanding is that most of those stories involve aluminum frames and probably aluminum rivnuts. I'm not expecting any problems from my steel/steel application.

Other than a drill, a small pilot bit, a 19/64" bit, and a little grease, this is what I used. That's an M5 bolt (I grabbed the clamp bolt from an unused brake lever), a 10 mm nut (you do NOT want the nut thread to fit the bolt) and a couple fender washers. The wrenches hopefully are obvious.

After applying some grease to both of the washers, the bits are assembled finger tight as shown prior to inserting the rivnut into the hole.

The 10mm wrench holds the nut while the bolt is turned to draw the rivnut tight. When it suddenly becomes more difficult to turn, it's time to stop.

The result is a well-crimped, solid attachment point for bottle cages. It might also prove useful for a chainguard-to-seat tube attachment. I wouldn't use them for smaller tubing, though - eg. seatstay rack attachment points.