I've had my 2005 mazda6 s for 10 years (106k miles), bought it used as a 1 year old for $17.6k. I love the car, but it has a real problem for longevity in MI. The car will rust on the rear wheel well lip, typically in the passenger's side. This is not just bad luck with my car, its a defect across most of these cars. If you see a 2003-2005 in the midwest, odds are it will have rust in this exact spot. There are about 6 of these cars in the parking lot where I work, and all but 1 of them have this issue. Aside from that though, this car has been very solid mechanically, with almost no issues outside of routine maintenance. It's fun to drive, I get 19-25 MPG depending on weather (and I'm a somewhat aggressive driver), and I think the interior is laid out well and comfortable. Its a real shame that the body isn't going to hold up long enough to match the drivetrain. Also, it handles terribly in the snow with stock tires, but does quite well with snow tires on.
I have owned my 2005 Mazda 6 for two years and she is an amazing ride, with a great heart (engine) and excellent get up and go. Unfortunately once I hit 160k regular maintanence didn't cut it. $2200 Later: struts, control arm, tires, headlights, tailights, stereo lights are burnt out and the shifter lights are dead. I love this car but the older she gets the more she costs.
Going into my fifth year (at 92000 miles) with my 2005 Mazda6S GT, I'm still enjoying the driver oriented experience baked into this car from day one. Only reliability issues I've encountered are the headlights burning out and a strut bearing causing noise.
Otherwise, taking good care to replace general maintenance items has kept the car in good working condition, and I don't baby the car. The relationship between driver and car, in regards to seating position, steering, throttle, and general feel is fantastic. Refinement could be better (2006+ models are noticeably more refined), but I enjoy this car every time I drive it. New sport shocks, springs, and larger rear sway added.
The 2.3L 4-cylinder engine found in the Mazda6 is completely adequate for most people, but when combined with the body of the Mazda6, causes some noticeable deficiencies.
1) The body weighs too much overall.
This leads to mediocre acceleration.
I will say, however, the speed and power available on long uphill grades is above average.
2) The body is low-profile.
This means you can't put standard chains on the front tires.
Only cables or low-profile chains (which are extremely expensive).
3) If you drive much past 90 mph, the downforce winds and centrifugal force on the tires cause the tires to rub against the wheel wells.
The engine is capable of more than the body can take
This is my primary business vehicle as a traveling sales manager with a long commute. I purchase a new or almost new car every three years and because of the high mileage and business use I prefer mid-size sedans that don't kill me on depreciation. My 6S satisfies my desire for performance and handling. I opted for the five-speed which I am glad I did because the engine is a bit weak until 2500 rpm then revs smoothly and quickly. Great cruiser, reliable, fun and 23-27 mpg