I bought these on sale in small size for 1-1.5hr indoor trainer sessions on the TT bike in aero position. They do seem more comfortable than wearing bib shorts and of course a little cooler. They do look like you've wet your pants though after an hour or so riding!
I purchased small and am ~ 69kg, 174cm, 79cm waist and ~90cm hips. They fit very nice and snug and don't move around but the waist is really tight, but looks like I am between sizes. I fold them over onto my hips and they are fine. I have a feeling the next size up - medium - could be a little loose below but would be interesting to try. I'm tempted to buy again as at half price they are good buying.