Long story short, in my opinion the AR-15 is a specific case where a "brand premium" is not worth it, most of the time. Here's a few reasons why or why not:

• The manufacturers are all trying to build the exact same thing, and follow the exact same mil-specs. This results in guns that are almost entirely identical, with only small differences in "fit and finish". What would the end results really look like if both Cadillac and Kia were following the exact same plans to build the exact same cars?
• The difference in prices between economy and premium brands can be significant (+50%). Even if there were minor measurable differences in accuracy or performance, would the overall performance result be better if you bought the economy model and spent the extra money on training, practice ammo, optics, or upgrades? (Probably.)
• AR manufacturers should be more appropriately called "assemblers", as they are rarely making most of the important parts themselves, if any. There are a limited number of true manufacturers of parts (e.g. upper/lowers forges, barrel makers, stock molders, etc.) so many manufacturers ("assemblers") are building what are essentially the same rifles, from the same parts, and in fact they differ only in "fit and finish" and quality control.
• There use to be a nice comparison spreadsheet floating around the internet that compared many of the popular manufacturers, though it has since become obsolete and been removed. You can still find screenshots of it with Google to get an idea. My impression was that the key differences were largely quality control (e.g. magnetic testing of chamber, etc.), features that are now almost universal (e.g. M4 feed ramps), or are "optional" or at least less important for civilians (e.g. chrome lined barrel, military/commercial buffer tube size, 1:7 or 1:9 twist barrel, etc.). Here is a link, but note that this is EXTREMELY outdated, and even economy models have many more features now: http://gunfacts.webs.com/M4Chart1.gif
• Manufacturers that are using CNC machined uppers/lowers are likely doing it themselves, so quality could be all over the board. There are no mil-specs for CNC'd uppers/lowers, because the military's are cast/forged. Also, the CNC'd versions are almost always unique designs to that company, intended to be as compatible with the mil-spec versions as possible, but typically incorporate extensive functional or aesthetic design changes. Since they are pushing the edge, these could be spectacular enhancements or incompetent garage-shop work. Pay careful attention to what you are getting.
• There are some changes to the AR-15 platform that will causes significant price differences. The most notable example is a piston driven gun (instead of the standard Direct Gas Impingement [DGI] system). The addition of a piston requires non-trivial modifications to the gun, and all the manufacturers have their own proprietary way of doing it. Early attempts to convert AR-15s to run with a piston had some long-term reliability issues, that have largely been worked out now. However, there are still some budget-minded piston kits available that use the older designs. Expect a piston AR-15 to cost 30-60% more than a DGI equivalent (e.g. $1500+ vs. $700+). One notable "sleeper deal" on a piston AR-15 is the Ruger SR-556e ("e" for "economy") model - at about $1100, it is a great buy considering the features. Overall, the safest recommendation, if you are in the market for a piston drive, is to buy a gun that was completely designed as a piston drive from the ground up, such as the FN-SCAR, Bushmaster/Remington ACR, ADCOR BEAR, etc. (all of which are up in the $2500+ range).
• You aren't going to "go wrong" with a premium AR-15, though.  You'll just pay a lot more, for a few more features.  It's a great example of the Law of Diminishing Returns.  A $1500 gun may be 10% better than a $750 model, but costs 100% more, for example.  If you demand the added features, then there is no question that you need the better gun to meet your expectations.  If your need of those extra benefits is questionable, then the dramatic price increase is likely not worth it.  I have also personally seen $650 economy AR's out perform $1200 premium models.