I find them very useful. The KTM carbs are hard to get "perfect." The FlexJets help a great deal and give you a chance to try different idle settings without having to remove tanks to get to the the carbs to turn the idle screws. Also, I live at 850 ft. above sea level, but I try to go over to Colorado at least once a year. I found that I needed to turn the idle screws down just a quarter of a turn to get the idle just right. I also ride in temps up to 100 and down to 20. I find this bike does need minor idle mix adjustments at either extreme. Again, the KTM carbs are just very picky.
All in all, I find them very useful. But others comment are correct; if you get your carbs set and never leave the same elevation, you might not see the advantage.
"I'd like to meet the joker who had the nerve to call this a road!" -- Walter Sigmann
"Gravity is a very fickle mistress." -- Unknown
"Throttle to the donkey!" -- Marc Coma