I see the custom market as risky. The custom might just be built for a single person. You get something you like, but maybe no one else likes it.
It''s a bit like buying a custom suit. It fits you perfectly. It might not fit others perfectly but no worse than them buying an off the shelf bike (unless your proportions are completely crazy). The other option is to buy an off the shelf suit your self, which will likely work out just fine too, but is very unlikely to fit perfectly.
And as far as the Rivendell vs. Rock Lobster thing goes... there are very different build philosophies between the two. Both companies built nice bikes but there is little or no overlap in their products. Paul (at Rock Lobster) tends to specialize in racing frames (although I'm sure he'll build other bikes as well). Rivendell specializes in bikes that are not intended to be raced and are designed more for touring and exploring than for going fast. Again, both bikes are nice but completely different.
Each bicycle owned exponentially increases the probability that none is working correctly.