Outside of BUILD i've made them using models with animations and linking the camera to the skull bone. Creates an amazing effect, but sometimes the use of it can be done badly, like making the movement sluggish as hell and the transition between running and walking or crouch and jumping SLOW. A lot of games do it real well, like Halo. The model itself doesn't effect movement but the fact that you can see your feet gives you a bit more feeling of awareness.
One game that does it okay is Dark Messiah, that game's movement and everything is attached to the FBA system....though its sluggish at times and feels unnatural since we aren't given the feeling of muscle tension and so on.
in terms of doing it in Build....I think its pointless. You can't look all the way up and all the way down anyways (well at least by default) so you never notice it. If you are doing it for movement sake, you can easily use CONs to make the movement seem a lot less mechanical and natural by simulating direction and how fast you start moving in another direction depending on the angle change. Who knows, the idea is there, but is there are reason to use it. Your choice.
Off topic: Here's a tech vid of my FBA for a game competition. (didn't finish in time, to big of an idea based on the deadline). Suppose to be a browser game with first person mechanics and immersive techniques while fitting the 'casual' market. Also has a subtle idle breathing animation.
http://www.facebook....v=1789770352473
let me know if you can't see it.