How do you say "what of good use are you to me?"?

Either ask:

"What good are you?"

Or ask:

"What use are you?"

You'd either say "good" or "use," not both. Whichever way you say it, it expresses the same idea. Also, "to me" is implied, so you don't need to say it. 

You wouldn't ever ask, "What of good use are you to me?" That's not grammatical. While sense can be made of it, meaning people can figure out what you mean, it doesn't really make sense. It's broken English.

By the way, this is a question you ask rhetorically to suggest that the person you're speaking to has nothing good to offer, is useless, something someone says in frustration after the person they're speaking to makes a mistake or after the person they're speaking to says that they're unable to do what's been expected of them.