The young leaves are best and have a cool cucumber taste. The flowers are lovely and edible. I have used them in salads as well as freezing them in ice cubes. I gave my Mom this hint, so for a Club meeting she was hosting, she picked some Borage flowers and froze them the ice cubes.
However, I had failed to tell her to pop out the seeds, so when she served the drinks with the frozen borage flowers, her ladies shrieked! The seeds still frozen in the borage flowers looked like frozen spiders floating in their tea!