We grew 3 varieties this summer.
Red

Green

And a red/green mix

It grows really quickly, new young leaves are soft enough to eat raw in salad so it makes a good replacement for lettuce in salads and doesn't have a strong flavour so won't overwhelm a salad mix. Older leaves have a firmer texture and as @baragua said can be used as a spinach replacement, you literally cook it the same way you would spinach. Blanche, plunge into ice water and season with salt, olive oil and lemon juice or ponzu and top with katsuobushi flakes, chopped peanuts or chopped cashews. The leaves have surprising levels of vitamin A, C and K and respectable amounts of minerals as well.
As the plants are so vigorous we're also using it as chicken food and the birds seem to love eating it, presumably the red leaves will contribute to the colour of the yolks. You can probably start harvesting at around 3 - 4 weeks after planting so there's still time to get some planted for this summer.
They also produce prodigious amounts of seed which can be made into a porridge or can be made into Amaranth popcorn
