I have two custom tourers, one of which was built specifically for the Rohloff. They are both wonderful, but both extremely different in ride quality despite being customized to me, my body dimensions, and to my rather similar demands between the two.
I'm sure it's been said here, but if you buy a custom, before you even make call number 1 to the builder, make sure to educate yourself, and take the necessary time to do it. Build your dream bike in your mind before you do anything, and be damned thorough about it. It's a major decision, and the end product is only going to be as good as your concept. That's the point of a custom. This absolutely includes all component choices you intend, not just the frame. As for the frame itself, there are so many choices to make, so many choices that could have been made and yet aren't on the ordering sheet, and so many things you might do differently later on and might regret, etcetera...
Here's the way my experience has gone so far:
2005 Mercian King of Mercia tourer
Okay, this is my best bike, maybe the best bike I've ever seen according to my own tastes, the dream, etcetera... The concept was two-fold: I wanted a bicycle that would split the difference between being both aesthetically beyond gorgeous and functionally beyond practical. After research, I decided I wanted it low-maintenance too, so I made the decision to have sealed bearing everything, highest quality components, and a Rohloff hub. One good thing about Rohloff is that it really is at its best on a frame that is built for it. I had mine built with the sliding dropouts instead of the eccentric bottom bracket, and chose the OEM Rohloff hub with the internal gear mech. All I have to say about the Rohloff is that it is incredible once you break it in.
Here's the list of stuff I put on it. It is notable that the list comprises parts from 7 different countries. This is almost everything:
USA: Paul Neo Retro brakes, Paul Touring Canti brakes, Cane Creek SC-5 brake levers, Chris King headset, Phil Wood bottom bracket, Phil Wood low-flange front hub, Velo Orange retro bottle cages
Germany: Rohloff speedhub, Wippermann chain, Tubus Logo rear rack, Tubus Tara front rack, Schwalbe Marathon Supreme tires
England: Mercian King of Mercia frame, Brooks saddle
Japan: Nitto 'Dream' handlebar, Nitto Technomic Deluxe stem, Nitto 'Crystal Fellow' seatpost, Shimano SPD pedals, Honjo 'Le Paeon' fenders
Italy: Campagnolo Chorus crankset
France: Mavic A719 rims (32 hole, as per Rohloff)
Switzerland: DT Swiss double butted spokes
Here's what it looks like:
Unladen
Laden
Everything about this bike was visualized in my mind beforehand save the fenders.
Changes I would have made: If you want higher handlebars, the place to start with is an oversized headtube, ala Rivendell. (I think the Surly LHT does this also but I'm too lazy to check). I'm fine with my current head tube, but making it oversized gives more flexibility if you *do* want to set the bars higher. I also would have gone with the 'Vincitore' model and its fancy lugs, but that is purely aesthetic.
2006 Bike Friday New World Tourist
Not much to say here. I needed a quick bike for a trip through Thailand, and I felt a folder would be perfect. I mostly went with their base suggested components, with not too many frills. As a custom everything went well enough and is a damned comfy and solid bicycle that withstood a long, long trip with a good friend. It's in its suitcase now, or I'd supply the parts to that one too, but I don't consider the "custom" aspect of this bike quite like what I did with the above bike.
Here's what it looks like.
Bottom line: I'm very, very happy with having made the decision on both of these. They are wonderful instruments of bicycling pleasure, and I recommend a custom dream bike to all men and women of action and variety. That said, and I'll just say it again, make sure you know what you're getting before you get it. Iron out all the details you can. Make decisions that best suit the meaning of what you're buying.
And I highly recommend Mercian as a builder, by the way. Wonderful quality and aesthetics, and maximum choice over what they do with it, all for the fairest price you can get in this type of bicycle.
Good luck!
-Matthew