My buddy has now had his 2006 black Impala 3LT 3900 for a year now as a fleet car for Fuji medical and gets to take it home every night for his daily driver. I have driven the car many times with him so have made the following pro and con table:
Pros: Spacious, quiet at cruise, lots of std features for cost, trunk space, performance, mileage, handling, workmanship, reliabilty
Cons: Anonymous rear end styling, engine noise at more than half throttle, hard seats take getting used to, no floor shift indicators or glove box light a minor annoyance
Now that the 06 Impala is fully broken in with 35,000 miles, performance and mileage have improved to quite respectful levels. We timed it with both a stop watch and G-force at 0-60 in 6.8 seconds with 2 of us aboard! The trip computer is now reading 24-25 combined mpg which is about a 60-40 ratio of highway to city driving which is an improvement of almost 2 mpg compared to when it was new. The engine has settled down a little and is a little quieter but still in our opinion makes more noise than it should. Still if your used to a 3800, you will love the 3900! It offers 40 more hp, similar combined mileage and a lot more driving fun. It's pure highway mpg may be slightly less but then again it has 3.29:1 gears vs 3.05:1 for all the 3800 Impalas. The seats are very firm but once we got used to them, they weren't a bother. The lack of a glovebox light was annoying at night and the lack of gear shift indicators made the center console look cheap. We also wish Chevy would put some padding in the door armrests like they used to. Rear seat legroom is slightly reduced from the 00-05 cars but worse, toe space is compromised because the seat comes almost to the floor from the covers. Ride and handling remain very good. This car can really pull around a sharp corner with the 17" tires. This car has only been in the shop once for a purge solonoid but otherwise has been trouble free with no squeaks and rattles, tight build quality and seamless interior fits. Overall it is an impressive car for the money and one that I can endorse to anyone looking for a medium to large sized sedan but go for the 3500 if you want steller highway mileage!