I've used one many times, usually with a 50mm f/2 or 20mm f/3.5, and found it can be used handheld if you are using flash.
I've also used an odd flexible clamp device to hold subjects in the wild. This has a small plate with screw to attach to the body's tripod socket on one end and a small clip/clamp ("roach clip" type) on the other. The two are connected by a stiff piece of bendable wire (copper ~8-12 gauge). I would clip a flower stem in the clamp and bend the wire to place the flower part at the proper focus point.
Reversing any prime between 20 and 50 works well. Several of the zooms (43-86, 35-105, ...) will also produce usable results when reversed directly on the body without any other extension.