As Aaron stated, a good builder can make a bike feel like anything with any material.

Steel has surpassed titanium in terms of weight. R&E cycles here only recommends titanium for people who want the aesthetics and longevity. Their S3 is 13 pounds, but man, that tubing must be thin. I just got a litespeed firenze, which gomango would probably pass up if he rode it. I haven't experienced much of the really high end titaniums so I don't know any better. Part of the joy in riding it is that it is bulletproof and will stand the test of time better than steel bikes.

As far as the joy:$ ratio, no one can answer that for you. If you are the kind of person who is always buying bikes to scratch an elusive itch, than a custom makes sense. If you are the average person who only rides every once in a while and has average proportions, then custom might not be worth the $ and an older bike might make more sense.

People who want custom have done their homework and know what they want. When I am out of school and am no longer paying medical school debts, then I will definitely spring for a custom.