I helped a friend assemble an AR-15 lower receiver this weekend, along with all the other stuff I did. Neither of us had ever put one together before.
A friend of mine calls them expensive jigsaw puzzles, and that's about right. With the help of instructions from AR15.com, we probably got it done in a little over an hour. A good part of that time was spent trying to force one or two little bitty springs and a tiny piece of metal- a "detent"- into their proper place. More frustrating than actually hard.
I'm not usually good with small complicated things, but I encourage anyone who's considered building their own AR rifle for defense, sport, or hunting to give it a try. I'll definitely build another when I have more money and time.