What you get in building your own is exactly what you want without wasting money in the form of buying a complete rifle and later changing out parts. The difficulty comes in knowing what you want to begin with; most of us end up reconfiguring or building other rifles as our needs or desires evolve.
Having said that, at the very least, you save the FET paid on complete firearms when you build your own. If you do so with a complete upper, you honestly do not need any special tools for assembling the lower (receiver extension nut can be installed without the special wrench). Putting one together will also help give you a better understanding of the platform.
As I mentioned before, if one of the PSA rifle kits offers what you're looking for, they cannot be beat for value. You get the rifle kit, then a stripped lower (that's the actual gun, the part that has to go through an FFL), and put it together. The rifle I showed, had I opted for a more basic lower and just used the included M4 style stock and grip, could have been put together for ~$900 including shipping and transfer fees. The rifle kit cost me $737 shipped, and PSA regularly has "blem" lowers for $75. Add $10 shipping on the lower plus FFL transfer fee (typically $20-$40). Last time I saw it in stock, that kit had gone up $80 from what I paid, but still a screamin' deal. The MI handguard alone is a $200 piece.
Alternately, if you don't like the offerings in full kits, you buy an assembled upper of your choosing (be sure to know whether it includes the bolt carrier group or not), and then either buy an assembled lower, or a stripped lower + lower build kit.
One of the reasons the AR is so popular is the ability of just about anyone to assemble from parts or reconfigure with minimal knowledge, experience and tools. The barrel extension design means that there is no messing with headspace when installing/changing barrels. They are plug-n-play. That is the biggest hang-up with most other designs, including the FAL, M1A and G3/CETME; barrel changes require more equipment and much more knowledge/experience to be done and done correctly.
