I own or have owned several factory complete rifles that include Colt, Bushmaster, RRA and few "parts" guns and I honestly have had better luck with my parts guns. That being said I've never had a major problem with any of my AR-15 rifles. There are a few companys that I would not use in a build up but if someone does there research they should have good luck.
I'm new to this board and have in the past been more of an AK guy where the quality of guns from country to country was based much more on reality than opinions. I've always owned ARs as well just wasn't "in" to them like the AKs That being said when push comes to shove alot of parts are from the same companys ( CMT, LMT, LAR, Olympic)and the metals used are from the same forge. I for one don't think you can go wrong with Bushmaster, RRA, Colt, CavArms, Armalite, StagArms, CMMG, Double Star and several I'm not thinking of. Also there are some great parts only companys like White Oak ,Leitner Wise and Mega.
If you really want the most Bang for the Buck pick the brand whos rep your most comfortible with go shoot the shit out of it. I will stand buy my opinion of building one though you just get to know your rifle better and it just adds a new aspect of a already great hobbie.