I love it, I use it as a special treat after a hard ride on cooler days usually. I either use some of their grain and soak it until it's all mushy (or I soak bran and use that). Then it's up to what they enjoy for the extras! I cut up apples and carrots into little pieces, and add some sweetness with either molasses or honey (if they don't like molasses). Sometimes--if I have it-- I add ground flax to boost their coat. Interesting to note is that dried garlic has immune boosting properties, if a cold seems to be going around the barn.
After I have figured out what all I want to put in it, I soak it all in hot water until it's mushy, but if you use hot water you have to be careful of the temperature before you give it to your horse so it doesn't burn them.
Even if there isn't so much a benefit for them, I like making mashes because my boys really seem to enjoy them.
Usually horses love simple apples, carrots :) I don't make by myself anything I much more prefer natural things ! I've never tried but it is said that horses love watermelon ! you should try and tell me :)