I've got a custom Seven. It's nice, mainly because you also spec the ride you want with Seven. They use different tubesets for various characteristics. For example, I wanted a stiffer rear that doesn't flex on steep climbs and sprints but still light overall. That added 0.3 lbs from their normal 2.3 lb in a 56.

Even though it all is custom, the dimensions are virtually identical to a stock Waterford and a Cervelo I have. So my conclusion is you pay a lot for custom and most people don't need it if you are just concerned about getting the right size frame.