The common stock isn't "worthless", it's the stock options to later-stage employees that are possibly worthless, or at least worth less than imagined. The lesson here is to never rush to judgement about the value of your options at a startup. You need to think about "how much
more will the company be worth when I exercise vs how much it was worth when I joined", vs the less meaningful "I have XXX dollars worth of options".
And yes, I am aware of the inherent discount for ISOs, so as an employee you still stand to make about 2/3rds the value of your options if the company is worth about as much as it was when you joined. This makes me a little suspect of the stated $15 strike price for ISOs, that doesn't sound like the typical 2/3 discount unless if the stock was privately worth $30-45/share at grant time.