I removed the rear seats this last weekend to help a friend move. I was quite surprised by how heavy they were. I felt a noticable performacne difference with them out -- it felt like the E was squirting about like a watermellon seed! Lot's of fun, much sportier -- reminded me of the acceleration with the manual but better (I've got the auto -- SoCal commute..grumble grumble). If I didn't have to carry my kid to and fro from daycare I'd leave them out all the time (I know -- they're one of my favorite features, too -- when I'm actually sitting back there). Not to mention the CAVERN of space available with them removed. It impressed everyone with the amount of stuff I could haul.
The acoustics sucked, though. Sounded like a cavern as well. Those rear seats help a ton in that regard (at least with the stock EX stereo setup).