We actually ran a closed beta program for a month or so with an invite system. We released invites slowly while we got the product up to speed, and then launched.
And, yes — there's more to going fulltime than hitting X customers. How much cash do you need in the bank to avoid cash flow issues? What's your margin like — is it really X users, or is it X*2? What's your churn rate? If you don't know the avg. lifetime of a customer, you may be out in the cold.
Am I naive to think "All I need to be sustainable is 100 paying users"?
Did you have a portion of those 1,500 users lined up before you started implementation? Before launch? How did you