I converted my daughters 20 in. bike to Presta using these and now I don't need to worry converting the frame pump on my bike to Schrader. One stem for everybody and one less thing for me to worry about. That's worth the price right there.
These do the job, but should be anodized for corrosion protection. I just had to replace a tube after a pinch flat, and could not get the corroded stem saver off the old stem...corrosion was so strong that the stem ripped out of the damaged tube, as I tried to remove the stem saver. I had to limp home with low pressure to prevent the new presta tube from extruding through the Schrader hole.
All I have to say is I'm glad I switched to presta valve tubes using these stem savers on my 1979 Takara Sport Road Bicycle, because I use a pair of 28mm Continental Super Sport Plus tires that require a maximum pressure of 115 psi.
They work, and the price ins't too bad, but they could be done better. The rim at the end made too much bulk around the valve hole where the rim tape is already doubled, and the thick stem to tube connection is. I slid the adapters on from the outside, and tightened down the nut on the presta stem to hold it in place. They could stand to be bit longer too. They come shrin wrapped to a card and the plastic is very difficult to get off, I had to scrape it away with a knife. Not bad product, but there's room to iprove.
I was trying to use tubes with schrader valves on my recumbent trike. The front rims are 406 (20) size and my tubes bunched badly inside the tire. Expected lifespan for the tube was short. Presta valve tubes fit my 20 rims much better. Since the tube is thin rubber and air pressure is 90-100 psi, the tube might blow out around the valve stem. The stem saver prevents this. It also holds the Presta valve stem straight during installation so the rubber on the stem can't be cut by the edge of the hole in the rim. This reduces chance of leaks, increases reliability.
I really like this item. By converting my shrader rims to safely accept presta tubes I can standardize and consoladate my inventory of spares tubes. I like these better than the rubber gromet style converter.