I have owned a 1996 Nissan Sentra for three years. So far it has not had any mechanical problems (knocking on wood). It has had the brake pads replaced once, but that is to be expected on a car with 90K miles on it.
The gas mileage is great, I generally go about 350 miles between fill ups. The ride is smooth, you can't feel most of the bumps on the road, and it is quiet, not much engine noise.
Size wise, it looks small from the outside but I have two booster seats in the back seat with no problem and occasionally we have had a third child back there too, without a booster seat. The trunk is pretty good sized, I can fit a double stroller plus about six bags of groceries in there all at one time.
The front seat is great for me, I am 5'3. My brother, at 6'1, does feel a bit cramped when he is driving this car though.
The small outside size is great for fitting into all of those tight parking spaces at the mall. So far this has been a great car for us.