This comes up all the time and it's silly.

If dropbox only had a high valuation on paper, but employees/founders/investors couldn't actually exchange that into real $ in their bank account then sure, you can say the jury is still out.

But that's not the case. They built a product, have tons of paying users, and have IPO'd with a market cap over $10B. It's not money that only exists on paper. Founders and employees can sell their shares for real money and buy real things.

Their shareholders aren't just a handful of VC's anymore - they are public. The jury has decided, it's successful