MMMM. It's one of my favorite things to do in the WORLD! My Man was the first I ever gave head to, and he was an excellent teacher (even forgiving the few times I bit him the first few weeks I tried it MANY MANY years ago)
The change is firmness and texture, the way his balls pull up right before he comes, the little pre-come that drips our before, YUMMY! If I have already had at least one orgasm I can almost always come some more just from blowing him.
My favorite position is lying on my back, with him kneeling over me. It's probably a combination of "subbiness" and laziness. When we dabble in power play, he won't let me have it right as I really start to get into it, and I have to beg for it, and he says things like, "You can have it when I say you can." Jeez, the self control this guy has at times.