Yep, they're supposed to be bitter. But I agree, some, like the fatter-stemmed ones, are more bitter regardless of what you try to do to them.

Don't throw the stems out! They are so good! You need to trim and peel the legs sort of like asparagus (if you have the patience and time). Then blanch the legs, followed by the leaves, then saute all with garlic/olive oil, s+p. Sometimes I don't even bother with the blanching when the rabe are very young.

One of my favorite ways to prepare broccoli rabe is to steam it, then warm it in a pan with a little creme fraiche (or just cream) and lemon juice and zest. The cream tames the bitterness and the lemon livens up all the other flavors.

Could I just say up front that broccoli rabe is supposed to be bitter. Unlike Americans, Italians actually relish those bitter flavors which you will find in radicchio and cicoria. I've cooked some jobs here where I was vexed to see uneaten radicchio parts coming back to the kitchen. It's a cultural thing I suppose and a matter of taste. Italians like both sweet and bitter but maybe not in the same mouthful.

With broccoli rabe, you need to take out the stems. I wouldn't suggest boiling them, but rather slowly sauteeing them with garlic, olive oil, and chili flake. A good amount of salt as well. Broccoli rabe inevitably is bitter, however, I feel blanching won't remove the bitterness. Sauteeing also allows for browning on the edges, which will give a crisp bite to the vegetable. Also, purchase younger broccoli rabe; older stuff is just bitter.