I pay, I think $10 a month? It's worth it. Many people will probably grow into paying for it. It really is a nice product. The sharing function is the killer part, not the backup. People new to it might not have discovered how easy it is to share files with it, and then your Dropbox can fill up pretty fast.

Agree that the current accounts probably aren't worth $40/each - the valuation includes the technology, brand, and growth potential of course, not just the current userbase. But a lot of people will pay. Really, sharing big files used to be a major pain point online. Dropbox makes it ridiculously easy.