"Other than the joy of building your own AR15, whats the benefit? "
Why do we have to discount the joy aspect ? I didn't care in the least about the cost of it. I would have built my first one if it cost twice as much. I wanted to build one just because I wanted to build one. If cost is an issue, I know I can build one for less. I found a guy on AR15.com that was selling lowers for $79 each. These were forged lowers. Out the door they ended up costing me a little more than $100. A Bushmaster lower receiver parts kit is $64.95 and a stock is $74.95. So they cost me very close to $239.90. A complete lower with buttstock from Bushmaster is $295 plus the dealer vigorish. Putting one of these together is not rocket science. In fact, as far as I am concerned, the ones I put together couldn't have been put together any better by the finest gunsmith that ever lived. These are military rifles, designed to be easily repaired. There is no hand fitting of parts, there is no fine finish; the parts are just assembled and there is only one way to do it. The upper receiver tends to lose out financially because of the tools that you pretty much have to buy in order to do it right. Although, if you don't consider the tools, you can certainly save money putting together your own upper. In fact I priced one out last night. Realistically, the only way you could not count the price of the tools is if you do enough of them to make it worthwhile, or you borrow the tools, or you split the tools with someone else. The upper I priced out last night was for an older pre-ban Colt. They have different pin sizes than any other manufacturer. So, there arn't uppers lying around on every street corner that fit your lower without using adapter pins. And even if you find one, it might not be the configuration you want. So, you build it yourself. For example; I wanted to put together an AR15 that was a clone of the rifle I was issued in the military in the early 80s. This would have a forward assist, no brass deflector, 20" pencil barrel, bayonet lug, and triangular handguards. I have never seen such an upper advertised and if I wanted to build one, I couldn't use the upper receivers currently produced because they have a brass deflector. So, I was at a gunshow and spotted a used upper with an 11.5" barrel and 5" Flash suppressor. I wasn't really interested in this configuration, but the upper receiver was exactly what I needed, so I bought it and took it apart to use to make the upper I wanted. But the real reason I do it is because it is a neat project to work on. Although I was kind of disappointed because it was so easy, the project was done in a couple hours. You learn all the ins and outs of the rifle. If you ever have a problem, you know how the rifle works and how to fix it. And you can build it the way you want it. You can buy the parts you want, and you don't have to buy the parts that you don't want that you are only going to take off. I have a whole pile of handguards from factory rifles that I have removed. In a couple cases I thought it would be cool to have green furniture rather than black, so I now have black pistol grips and handguards lying around. In another case, I bought a Bushmaster M4. Unlike the real M4, it came with the small carbine handguards. I wanted the real M4 handguards. So I ended up paying for a set that I never used. Yet another case, I wanted a Knight's Armament RAS II. So, I removed the handguards that were on the rifle and replaced them with the RAS II; another set of handguards that I paid for that are now in the pile. I have another rifle with a Surefire 500 series weapons light on it. This is a flashlight unit that is built into a set of handguards; another set of old handguards lying in the pile. Maybe you want an Ace Skeleton Stock. Why should you buy a complete rifle, take the stock off and buy the new one when you could simply buy the stock you want and put it on. I have a Colt Lightweight Sporter in 9mm. It is a pre-ban rifle and I wanted a collapsable stock on it, so I now have an A2 stock sitting here in the pile, which again, I paid for when I bought the complete rifle.
"Having a good manufacturer assemble it (I feel Oly is good) offers so many benefits that you lose when you do it yourself."
What are these benefits ?