They can be bitter, but I like to make pesto with them. If they are too bitter I add a little honey. My grandmother would ruff chop, fry them up with a little olive oil and garlic. She would serve them one night and take the rest and whip them into whipped potatoes (it would turn them green and us kids thought green potatoes, WOW). She would do the same thing with spinach some times. Most kids don't like light fried greens but they like them hid in green whipped potatoes.
When I do roasted vegetables I will toss some chopped tops on the last few min. some times to give it a little green flavor.