I had a lovely pair for cross country - they were synthetic sandwiched in leather, a little thicker than plain leather ones but not uncomfortably so and they were just as they said on the tin. (I sold them a few years ago and as far as I know they are still going 20+ years later.)
I have Devoucoux ones which are brill and they definately work. They are buffalo leather so are very soft too. They are a little pricey but well worth it.
I bought a pair of prestige about 4 years ago - fantastic! The leather is so soft. I now have butet which are great as well. I wouldn't use anything other than the prestige or butet now, they are definately worth the money.
I have Jaguar ones which are synthetic inside leather. Really lovely, soft and the buckle is shaped so it doesn't dig in to your leg. What I really like is that the holes are close together and they come in lots of sizes. Well worth the money and definately work.
Synthetic on leather can mark it quite badly.
Another who loves Devoucoux leathers. fabulously soft, never had a pair stretch and have holes spaced at half inch intervals, until you have leathers with holes this close you cannot appreciate the difference this makes! Also currently have some Jeffries calfskin ones which I hate as they are like strips of cardboard and have had Jaguar ones in the past which were OK but not as nice IMO as the Devoucoux.
I have Prestige ones picked them up in the USA as half price! Only paid £30 for them. A very kind friend bought them back to UK for me. Well worth looking on USA sites as dollar so weak.
I have had synthetic ones for hunting and they were rubbish biggest problem is they do not adjust easily as do not slide as well as leather.
I also tried Cotswold Sport ones and they were fine but too thick for my saddle as are treadstone. On an ordinary saddle I think they would be fine but not on my monoflap.