My favorite thing about the Prius (other than the economy) is the simply MASSIVE space inside -- particularly rear seat legroom and the flat floor, but the trunk is huge as well.

My least favorite thing is the near-deafening road noise, particularly in the back seat. It's got a tin can door closure sound that reminds me of a Renault 5 or something. It's a HUGE step down from most economy cars in those regards.

This was through the 2nd and 3rd gen, at least. I've not been in a 4th gen.