You should understand the tax (particularly AMT) implications of exercising. $20k cost to exercise. $300k to Uncle Sam (given your 50x increase).
Very few articles on ISOs and AMT highlight the minimum tax credit that is applied when the amount paid under AMT exceeds what would otherwise have been paid.
What if you exercise a little every year, triggering AMT each year? No credit for you and the credit is reduced the older it gets. Like you said, each situation is different, but most people are going to pay a boatload in taxes on this and not be able to claim it as a credit later on.