What happens if/when you leave before all your ISOs have vested? Do you forfeit the exercise price for the unvested stuff, or does the company typically refund it?
the company refunds it. technically, what you generally agree to when you early exercise is to give the company the right to purchase the unvested shares back from you at the original purchase price.
The caveat is that you lose whatever taxes you paid on exercising unvested shares. Gotta be really careful with that, as the taxes could cost way more than the exercise itself.
Sure, but there are instances when that's not the case, e.g. early exercise implemented by company after the fact, or holding off buying until you're reasonably sure that the shares won't tank next month.