I have had Wilson Staff and Adams Tight Lies GI clubs since I started, and recently bought a set or irons from Gigagolf (Pursuit CBs) and another from Diamond Tour (Z-Force Z-35 Ping G30 clones).
Firstly, the head. The Z-Force Z-35 are quite nice, they are as easy to hit and (as far as I can tell having tried the OEM version for 1 round) they play very similar to the brand club they are trying to immitate. I certainly can't tell the difference, neither of them is incredible (sorry if I disagree with GolfWRX on this one, I can't see the Ping G30s going down in history as clubs which changed the face of the game), both are very good GI clubs. The Pursuit CBs are another category of club, they were more expensive and in my view totally worth it, from my perspective they are as good as any top-notch model from a top name, definitely superior in finish to entry-level OEM clubs, as far as I could tell (also not revolutionary, they just claim to be a classic style of cavity backed forged irons, and do just that, to a high standard of performance). So overall, you need to compare entry-level clones with entry-level OEMs, and in that respect the OEM club will win. If you compare top-notch OEM with top-notch clones, likely the OEM club would win too. However, you're paying 3 times as much for the OEM club, so for my part I would be looking for something MUCH better, which I am not getting. If you compare on a $/$ basis, the top-notch clubs from the best component brands are, in my view, much better than the entry-level OEM clubs, for a similar price. I do recommend spending time going into the detail of the materials used in making the clubs so you know what you are getting into, as both OEMs and component manufacturers occasionaly sacrifice quality to protect their margins or go for lower costs... You will be surprised to see how many OEM clubs are made with the cheapest stainless steel alloys, even sometimes worse than component manufacturers.
Second, the shaft. Actually this is a much more important feature than the clubhead itself when building a club. In this respect, if you're buying off-the-shelf (or even worse, off-the-Ebay), you typically don't get to choose. If you don't like the (usually mediocre for entry-level clubs) shaft which the OEM brand has picked, then you just have to go out and get a shaft suited for you, thereby paying for a second shaft. Unless you can build the club yourself, it is likely you will have to pay somebody to do it for you, and in the process you'll probably have to buy a ferrule, some fitting equipment, a grip etc. The key difference with the component company is that most good manufacturers give you a wide range of choices of shafts, they will build to your specs and will fit for a very reasonable price. So, instead of tinkering and paying double to have the shaft you wanted in the first place, you get it right from the word go. So in this respect, the component manufacturer offers the same or better equipment for a fraction of the price.
Third, the grip. Very similar to what happens with your shaft, if you don't like the grip on your off-the-shelf clubs, you're screwed and will end up paying for another, whereas the component manufacturers let you choose whatever you want in a wide range. In terms of wear and tear, replacing grips is necessary from time-to-time, and you can be sure the grips which your OEM brands pick are not made to last, on the contrary. They want you to come back for more, so they choose soft plastic/gum which is very sticky and soft at first, but soon wears out and needs to be replaced. If you want to keep your costs down (especially if you tend to play in cold and wet weather which will cause greater wear and tear), you might want to go for a chord grip which will last longer, therefore costing less over time. Also in this respect, you get the same or better equipment for a fraction of the price.
Fourthly, the assembly process. Unless you're Tiger Woods, Nike clubs you buy off the shelf are rarely going to be fitted for you. If you buy the top of the range clubs, they might fit them for free, but not if you buy the US$500 set advertised on Golfsmith, if you get a bargain deal at Galaxy or if you buy second hand on Ebay (even worse, the latter may have been fitted for a 5" hitter with a flat swing and it so happens you are 6"4 and need upright adjustment). Most component manufacturer offer some degree of customization in the fitting process, including lie and shaft length adjustment at least. In some cases, they can also adjust swing weight, loft, grind, spine alignment of shafts, puring of shafts, frequency matching or other niceties which can add to the quality of the fitting. When you have tested the difference between clubs which are properly fitted with this level of attention vs any OEM off the shelf club, there is no way you can go back to off the shelf. In my experience the quality of the fitting for "cheap" clubs is much, much better at component companies than at the OEMs. In fact, in most cases you are getting almost the same service as Tiger Woods at Nike, which is just awesome.
Finally, the customer service. I spend about 2-3 week-ends choosing my new clubs, trying them out, going through variations of fitting options with the fitter. The guy at my local golf warehouse is always trying to lead me towards the most expensive, newest gimmick from the OEM brands, getting me to spend those extra bucks that go straight to his commission. In my experience both with Gigagolf and Diamondtour, those guys are in it for the long run. They want you to come back and buy from them in the future, so they give you real advice, even if that's at the expense of selling you that top-notch driver from their line. They will work with you in the process, not against you. For me, that made the whole process of fitting much more friendly and amenable, and if only for that, I will definitely go back to them.
So, for my part, I will continue to pay 1/3rd of the price for a component brand club which fully satisfies me. No doubt the pros on the PGA tour will survive without my money coming to them via a sponsorship deal with one of the OEM brands.