Have any of you finished an AR lower, and if so how easy/difficult was it?

I have thought of getting a couple of these as a challenge (and because they are easier to find), but only if there is little to no chance of screwing them up because they are expensive enough <img src="/ubbthreads/images/graemlins/tongue.gif" alt="" />

The one I'm specifically looking at is the American Spirit Arms 95% lower.

I have never built an AR at all, but how hard can it be?