I see two reasons to go the 80% route. One is that you don't want a paper trail. The other is you like the challenge.
The latter is the correct choice. There was also a time when you could save money this way, but that's about four years past, now (in 2012 it seemed like there was a glut or parts and shortage of rifles, stemming back to the 2008 panic, before Newtown when everything disappeared --lot's of semi-auto subgun conversions use AR firing pins, and there was a critical shortage of those for months!)
If you wish to know your rifle better than any training or manual could possibly teach you, build one, and figure out how to make it work
Regarding the ATF's list of FAQ's about 80%'s, there's a couple things I take issue with in the list (bear in mind that nothing in the list is or needs to be legally binding, not subject to change, or not in conflict with past or future rulings), namely that they seem to imply you need a serial on the gun in order to sell it, which is bunk. You simply can't make a gun with the intent to sell it (or more importantly, the appearance thereof. See post 2 for an example
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Regarding milling with drill presses; non-machinists are not aware that not only is this activity rough on the drill press (it's bearings aren't built to take cross-loads), it's also potentially quite dangerous. Most chucks are wedged in place by the purely axial force of straight drilling, but milling puts side loads that can suddenly pop the chuck free from the spindle while running. Depending on your proximity and how much force is going into the cut...flying chucks, exploding endmills, and bears, oh my! Stay safe.
TCB
(who got his vewy fust dwill pwess today; little benchtop model that'll drill small holes straight(ish) and not much more)