With Walnut, it makes a decent enough pipe, but when your cutting and sanding it, make sure and wear a dust mask as the dust from Walnut is very toxic from what I understand. Hard on the lungs.
Of course, you should do this with any wood unless your using a good dust collection system.
I've made a couple of my own stems and you should know, you can put almost as much time in a handmade stem as you can the making of the pipe itself. So for now, just learning pipe making, I'm using premade stems, although I do reshape them and also add delrin as a tenon.