I'm a custom builder.
Despite that I'll say that your result will only be as good as the decisions made about what you need. If you pick a builder with ideas that don't match your needs, it's pointless. Same goes for custom fittings or having the shop employees choose a stock size for you. If they choose wrong, the results will be bad, so it's important to do your homework and learn everything you can about what you truly need so that you can evaluate and 'fact check' your potential builders.
You can pick a custom builder for fit, features, aesthetics, and handling. Not everybody has specific needs in each of those categories. And some people just want something unique that they had a hand in designing, that's valid. I encourage people to do a rational evaluation of what they actually want and what's the best way to achieve it, even if it's not a custom frame.
Most customs end up pretty close to production geometry, so there's always someone who'll want a used one. New bikes are only a good investment when you use them. Your body and mind get the returns, not your wallet (;