Agreed. I actually got introduced to it by a client who you wouldn't expect to use a startup's product. Fastest I've ever gone from free account to paying customer.
I'm not a customer yet but I agree on it being indispensable. It'll be an easy decision to pay once I run out of disk space. There aren't many products which I actively promote to my friends, but dropbox is one of them (the extra space for referrals helps too).