To elaborate on what Thomas is saying: they revolutionized rapid application development at both our companies, for like $15 each.

In consulting projects I often have call to create little one-off internal tools in Rails, either as a prototype for a tool the client will create or to support their marketing team. These generally have development budgets measured in days, and I have the HTML/CSS skills of roadkilled turtles, so they came out totally unstyled. I'm now embarrassed by them: now when I start a Rails project I drop in one of my prepared admin theme layouts and blammo it magically looks professional.

Given that my clients end up paying thousands for those mini-projects, the level of polish is clearly worth it. It's like a Tiffany box versus delivering the ring in a small paper bag: if you think it doesn't matter, your mental model of human cognition is broken.