My experience has been mostly on the now being phased out Ten Series, but the Cruise/Speed Match/Motion Match option appears to be unchanged. I have played with Speed Match and can imagine it being useful for somebody, but not for me. I usually kept the Motion Match switch set for the more aggressive setting, but now and then found the less aggressive setting more suitable, particularly when doing work on turf where it helped reduce tire skidding.
The most useful of the features for me has been the Automotive cruise. When using cruise it allows you to "resume" or return to the previously set speed after slowing or stopping. It also has a +/- switch that lets you speed up or slow down in stepped increments, similar to your car. With the standard cruise you have to find your speed and set it again everytime after slowing or stopping.
My experience is that the cruise maintains the speed exceptionally well as long as the engine isn't overloaded. When Load Match is turned on, it will slow the tractor to prevent the engine speed from dropping too much, even if cruise is set. I have used this combination while mowing and even once while running a snow blower and it works great. Just set cruise at the highest speed you want to travel, then Load Match will automatically slow the ground speed as taller grass (or deeper snow) is encountered and return to speed as the load lightens.