AR's are awesome. I just bought a Stag complete A2 lower half and then a Stag 1H complete upper half with the Plus Package. I basically made their model 1 carbine out of them.
If you buy the upper and lower half's separately, NOT assembled OR both at the same time and/or on the same invoice, you get out of paying the 11% federal excise tax. I have $844 total invested in mine including parts, shipping and sales tax. I thought about building one. I asked my FFL for prices and it seemed better for me to just go ahead and buy them separately.
My last AR, which was my first, was a Rock River Arms A2 Tasc Rifle 16" middy. It was a nice rifle as well. I can really only speak of these two as they're pretty much all I've had experience with. My friend built his own with DPMS parts. I shot it and it shot good. He didn't keep it to long so we don't know much about it now.
Good luck, NYH1.