As said above, it does depend a lot on the situation. My opinion is that you can certainly fit 3 adults in the car in reasonable comfort. The back seats are small and rather upright, but if the front passenger moves the seat forward there is enough room behind them. Whether a fourth adult will fit behind the driver depends on the position of their seat. I'm not that tall but like my seat back a bit, and it's pretty cramped behind it for an adult.
Don't let it put you off getting one. The reason they are small is to have such impressive boot space (with roof up), which is more than you'll get in an MX-5, Astra, Megane, SLK, and probably just about any other small convertible. And some of these have only 2 seats anyway. Although the roof uses a lot of the boot when down, the back seats certainly do give extra storage space.
So it really depends on whether you are more often carrying people or junk!
P.S. in short, same as Derek's reply, just more verbose!!
<font size=-1>[ This Message was edited by: Matt on 2002-05-25 08:00 ]</font>