As a guy who paid bills by building AR's for quite some time, I tend to recommend folks work with someone like me to build a custom rifle, but for guys looking for a turnkey option, this is a great rifle.
I have 3 Ruger AR-556's which I bought for use as loaners during "get to know your AR" training courses/classes I offer, and I've also used a few Ruger AR-556 receivers and their Elite 452 triggers in a couple rifles now. Their parts are on par with any thing else on the market, and typically slightly better at their price point. I do, however, wish I'd waited to order my AR-556's from Ruger, as I would have preferred to have MPR's with float tubes, rather than the clamshells, so I'll likely be selling off this year and upgrading now that these are on the market.
The Ruger AR-556, and now the MPR, are fine rifles for their price point. Mil-Spec Mall Ninja's get all up in arms about any AR model which isn't 100% mil-spec through and through, but there's a simple reality in our market - many of the hoops to jump through to be able to tout those 7 little letters, "mil-spec," really don't mean much for the civilian user, and frankly, might even mean you're passing on an even better option.
The Ruger 2 stage trigger is one of the better factory options on the market, and their barrels are proven to be some of the more accurate-for-the-money products as well. For their price, a guy can hardly justify building his own. Getting that rifle for $600 is a steal. A similar M-lok handguard from Midwest Industries runs something on the order of $150-180, the trigger itself is $140, a comparable trigger would be the Geiselle G2S-E priced at $175ish, as the Ruger is slightly better than the Rock River Arms 2 Stage Varminter or National Match trigger (slightly cheaper, something like $120?). Building that rifle for any appreciable amount less than the $650-675 street price would be a challenge - it could be built cheaper, but not much cheaper, and by the time you buy tools and pay shipping, you'd be more likely to spend a few hundred bucks MORE instead of saving $50-100.
Personally, I don't care for the MOE buttstock, so I'd replace the buttstock with a Magpul CTR, or better still, a UBR, then replace the buffer with an H2, and I'd shoot that Ruger until the cows come home.
If you change your mind and are interested in building your own, shoot me a PM.