For individual parts I get whatever is cheapest and I usually shop at Brownells.
It can be frustrating to pay that much for a parts kit when you already have some of the parts. Or, you plan on upgrading the pistol grip, trigger guard, selector, trigger, hammer, takedown pins etc etc. But buying the other parts can end up being more expensive that just getting a kit and not using some parts. I have constructed a simple table to help you make the decision, comparing the Brownells price per part with Anderson MFG price per part. I picked those 2 to give what I feel is a good general idea of whether to just get a kit or not.
Generally speaking, yes. Some example price differences are:
Brownells: Make your own kit out of parts individually - $100 From a Kit? $55
Aderson Mfg individually - $125 From a Kit? $66
However, if you have your own trigger group and you want to part out the rest of the lower and say, include a Magpul trigger guard and grip, then you can get the rest of the non-trigger parts for about $60 with the Magpul stuff, versus about $50 without. Still a borderline decision at best, but if you want anything else upgraded from that point, it starts to be better value to just get the parts individually. Assuming of course, you don't want to use the spare parts from a kit to start a new build...
What really sucks about buying lower parts kits or individual lower parts is paying for shipping, or there being almost every part you want in stock. Generally, a parts kit or any parts will work out cheaper if you get it while ordering something else to save on shipping. Or you can do what I do and just buy more magazines everytime you order a small part, but that can get expensive fast.
But thankfully, some vendors are offering semi-customized LPK's. Palmetto State Armory offers a large variety of LPK's with most of the typical upgrades you might want, like a Magpul pistol grip and trigger guard. They have 24 different kits right now.
In a word. No. They are not. I have picked up a lower parts kit as cheap as $35. In Vegas. They said it was from Colt but there was no way to tell and the trigger felt like it was dragging over gravel until I polished where it touches the hammer and disconnector. From Brownells a colt LPK will run over $130. Compared to less than $60. With no way of knowing if all parts came from the same shop or not.
I have never had a spring, detent, bolt catch or mag release, takedown pin or pistol grip screw fail me, nor heard of one that did. So unless there is an improved look or function to the part, just go with the cheapest available.