I had pretty much the same deal at my old job. Side note is that IIRC here buying the shares right away meant you had to pay taxes on the 'profit' based on the current stock valuation. That's of course lower than the price they'd be sold for later.. but also means if the company goes under you've not only lost the money you paid for the options but also a significant amount of taxes. That, combined with the very big risk of going under made me go risk averse and just keep them as options until an acquisition.

Of course for me it was moot because i got laid off less than a month before the options vested But the fact that they had to lay off half the company does nicely illustrate the high risks (that was after the first clinical study, even!).