I guess I distinguish between "factory customs", "prototypes" and "customs".
To me, "factory customs" are customs that can be ordered straight from Oakley and are made up from OEM parts e.g. Jawbreakers etc to suit your taste and desired colour pattern/lens. These don't really have much value to me, because there are an infinite number of permutations, that are able to be put together. I generally don't purchase these types of customs at all, unless I pick up a good deal second hand, cannibalise what I want and shift the rest of the frame/lens on.
"Prototypes" are desirable to me, because they are not widely available, and let's face it, it's cool to be able to get as part of a collection because they are rare, and simultaneously provide some history of the "finished product". In terms of quality of work, a lot are incomplete, don't have matching boxes, or even have markings on the frame or lenses. This makes it difficult to determine whether they are genuine or not, which is the problem with paying high prices for these. And this is the main problem most of us wouldn't be willing to pay top dollar for stuff like this unless we have a traceable trustable history with the seller.
Case in point - It's no secret that Romeo van Frogstein (Ashley) would love to own one of those blue or purple factory OEM Juliets, but when I talked to him, he raised a good point. Unless you actually have a proven factory one, it's hard to tell if someone just customised it themselves for $120 or whether it's worth the price of a "one of three" in existence. Everything is made by human beings, whether OEM or by someone who is non-Oakley, so who's to tell what the origins are? c.f. Fake artwork that floats around nowadays.
"Customs" breathe new life into old beat up frames. But they also provide "styles" in models that weren't OEM or provide a cheaper version of the original. Oak mentioned some of the non-OEM "Greats" - Brian, Zach, Josh etc. I have a number of customs from all of those guys which include Juliet rust decays, chameleon frogskin, RJ Gen 2 decay, stands that are glow in the dark, Atmos Holbrooks, gold chromed Jawbones, Gold Tintin 3D gascan with 24k, stealth black juliets and R1, Infinite hero new Racking Jackets, Holbrook blends, various cerakoted Juliets, a pair of polished carbon ichiro XS complete with etched slate lenses and linegear rubbers.
Beauty, of course, is in the eye of the beholder, and the purists amongst us would say that customs aren't really worth much at the end of the day as a beat up standard pair and that people always want "original". Well, yes and no.
It makes sense that if you alter something from factory, it probably won't be worth as much when you go to sell. But I've seen some customs go for amazing prices, because of the amount of work people know needs be spent.
My argument is, that in a way, the stuff these artists produce are in much much less volumes than commercial OEM limited edition pieces. And often, they'll be better quality. Look at some of the OEM rust decay and green fade stuff and you'll understand. Customs often they take longer to produce as well, and with definitely no less skill than the artists at Oakley. The guys who produce this stuff are good enough to work at Oakley. The only difference is that they're on the other side of the fence.
Customising allows us to get something that is truly unique. To me, that's worth something.