I like it, and think it's a very good method as long as no one close to you finds the mess. In fact, it's probably my second favourite method behind gunshot because it too is instant and if one does manage decapitation or serious injury there's no way of getting stitched back up.
Overall, I do think it's feasible to build one. Problem is faith and quality of one's construction. For instance, I have no idea how much pressure it takes to slice a neck, especially if it were to hit a vertebrae at the thickest point. Which is why I'd make several test runs and probably go heads up to make sure I don't paralyse myself without at least severing my throat. Also think one might have to compromise on the blade, because most of us probably have no idea how to make a proper one. So, I'd use a cleaver(s) or maybe even circular saws instead. Finally, while I find the method tempting when it works, becoming a quadriplegic is amongst my greatest fears, so I'd definitely use some form of back up. Maybe drink SN before, or better yet, build the contraption inside something enclosed like an empty swimming pool, flooding it during the procedure. Would also simplify the clean up. Heh.