The older design of the Impala's cabin doesn't affect the space that's available for passengers, or the trunk room offered up for their cargo. It's surpassed in fit and finish by newer designs, though, which offer better seats, finer materials and more in-car storage.
The Impala's seats have drawn comparison to those in GM's great Silverado trucks. They're wide and flat, without any side bolstering and with cushy, springy foam feel. That's good for truck drivers who need the extra damping from ride motions, but it doesn't feel as good in a sedan after hours of driving. Head and shoulder room are ample, but the Hyundai Azera and Toyota Avalon offer more leg room.
Impala LT and LTZ models come with folding rear seats that open up the trunk to the cabin. The trunk is already one of the larger ones in this class, at 18.6 cubic feet, though the trunklid cut-out steals a little space.
Overall, the Impala's cabin is quiet and refined, but it's a halfhearted attempt in the details, straddling the line between commercial-grade and retail-grade. There's a simple, straight-across dash, unimpressive finishes, and somewhat drab look to it all--although Chevy has upgraded the faux-woodgrain trim and added a leather-wrapped steering wheel and shift knob to some models.