I've got a sweet De Bernardi SL that I ran single-speed for a while, but it was destroying my knees, so I switched it to 1x9 for a while. I was using a 42 Ultegra chainring paired with a 12-27 cassette. However, I soon found that I was shifting more with the 1x9 setup than was with 2x9 on my other bike I use for commuting. My commuting route is very hilly, and I often shift to the big ring on big downhills and the flick the FD back to the little ring, so it involves relatively few shifts. With a 1x9, I was shifting up and down through a range of gears on many of the hills. Ultimately, I switched the back to a 2x9 with the classic 39/53 chainring and 12-27 cassette. It doesn't look as cool as it did S/S or 1x9, but my knees are much happier.