I will give you my own strong opinions about highend customs versus highend off-the-shelves (OTS).
1) If you're looking for a "race craft", the highend OTS (eg, Specialzed, Cervelo, Cannondale, Trek, Colnago, Pinarello, etc..) will in most cases beat the pants out of most SMALL custom builders, and yes, that include Crumpton, and easily beat the "one-man-shop) individual custom builders. The reason is very simple. The Big's simply have more input from their teams or riders and from the huge user base. They for the most part know what geometries work, and what don't. I'm always a bit baffled when I hear guys say that their so-and-so custom handles better than OTS bike.
Unless you have a very unusal anatomy, or a petite person, chances are, you can easily find an OTS bike that fits you in a way that will give you the handling you want. If you can't, then perhaps it's time to question your skillset.
2) Regarding value. This is where I believe customs start to shine. Think about this. A $10,000 mass produced bike somewhere in Asia will have a very LOW cost of production compared to another $10,000 bike produced by a custom builder or by a boutique builder (eg. Cyfac, Parlee, etc). In other words, the INTRINSIC value of the $10,000 mass-produced is much lower than that of a custom. So you are actually getting a LOT more value by buying a custom.
3) the craftsmanship. Most great customs (eg, Cyfac, Parlee, Serotta) will have the paint and finishes far more superior to that of a highend OTS (eg, Swork, Evo, etc). Custom wins in the craftsmanship.
I'm of normal proportions. I can pretty much buy any OTS bike with a 51.5cm effective top tube and make it fit me. But most of my bikes are custom because I believe in #2 and #3 above.
However, I'm not under any illusion that somehow my custom frames have that "magical geometry" that somehow enable me to handle the bike better than an OTS bike. I don't believe in any special geometry, because in order for a geometry to be truely magical, the frame would have to be tested on the rider, have the rider put in his real world input, then the builder would need to build another frame based on the rider's input. It's a refinement process that NO rider and custom builder will have the resource to carry out. Yet, I hear a guy getting a custom and immediately claim he's a better bike handler already. Wow, maybe his builder can go work for a MotoGP team and get the setup right the first time; MotoGP teams will be willing to pay him much bigger bux than he can get from building bicycles.
I generally prefer highend boutiques (Cyfac, Serotta) over a one-man-shop custom though.