Our 15-22 was a gateway to a M&P15OR. We were very pleased with both. I picked up the 15OR a few months back when CDNN had an increadable sale on them, so there really wasn't anything else at it's price that I felt was comparable (was $699 - $100 S&W Born in the USA rebate=$599).
I think "the chart" is a GREAT place to start, to learn what many of the main issues are; but sometimes I am reminded of the Zealots in the "Toy Story" Claw Machine chanting "the claw, the claw".

Sorry, but I am not stupid for buying a 5.56 AR with a 1:9 twist (true chart believers only accept 7:1, and often have harsh words for those who do otherwise

). 1:9 is pretty ideal for me, but if you only listen to those that chant "the chart, the chart", well, you needlessly limit your choices.
I'd like to "build" my own AR. But this being my first 5.56, my needs were pretty generic, so off the shelf worked well (I did get to change my gas block

). I think this has changed fast in the last year and is now like building your own personal computer.
PCs: If you are a hard core gamer, you build your own computer with all the best components for less than you could get a top end build for. But for average needs, you can pickup a fully assembled computer for much less than you can buy the components for (unless you are willing to invest a large amount of time bargain hunting, which can go wrong fast in one bad purchase or live in Taipei or certain cities in China).
ARs: With the way prices on fully assembled ARs have dropped the last 6 or so months, I think it's pretty easy to get a fully assembled AR for less than good parts. If you are willing to spend a lot of time looking for deals, you probably could assemble for a little less.
Now if you have specific needs, and know exactly what you want, it probably often winds up being better to build yourself. Especially if you have a list of really high end parts you want.