I bought a 39T ring to replace the stock little ring on my 52T/36T crankset. I live in an area with lots rolling terrain, and the 39T ring helps me hit the gearing sweetspot without inefficient crosschaining. I run a 11T-28T cassette and never feel undergeared on hills. Shimano says the 39T is only for use with the 53T ring, but mine works fine with the 52T; I've ridden this setup almost daily for ~2 months and only dropped a chain once due to bad shifting/pedaling technique.
After 11,000 miles and a couple of chain replacements another chain was due, wearing to .75%. That was more than my original front chain rings could bear and I was slipping on my large ring after the new chain was installed. I replaced my rings with these and no more slipping. In fact, what surprised me was the decrease in noise. It was remarkable. These are not cheap but they're genuine, in stock, and shipped right away. It was well worth it.
My bike had a compact set, which was just a little too low for my fast bike. I like spinning fast on my inner ring, but with a 34 tooth Id have to almost cross chain to go my normal speed. I bought a 36, and it is perfect. Now I can keep it more in the middle of my cassette and spin away. Good quality, worked perfectly swapping the 34 to the 36, with a 50 outer.