Well, I was actually gonna start a review thread on the '09 Mazda6.
I've not driven the RX8 so I can't say anything about it except that someone I know has one and it uses premium grade fuel, gets less than 20 MPG, but it is quick and handles well.
I've now driven both the '08 and '09 Mazda6.
The '08 is a tight little sedan with good power (even though the 2.3 is rated at a paltry 150-some HP) It is a firm riding car, but it will not beat you to death when the road gets bumpy. It corners nice and flat.
I have but 2 complaints: 1st. Mazda decided to back light the gauges in red. It grows on you over time. But I just find it unattractive, and hard to see at dusk, when it's still too bright to take off the shades, but too dark to run without headlights.
My other complaint has to do with the interior color scheme. If you chose a tan colored interior, everything from top of door up will still be black or grey.
So, for the fashion conscious, get a black interior.
The '09 I drove was a 2.5L with a 6-spd manual. It was a hoot to drive and 6th gear drops 500 RPM on the freeway at 70 MPH. The shifter was smooth, had nice short throws, and nice positive engagement. The clutch was smooth and progressive.
The 2.5 feels quite a bit more powerful than the 2.3, though there is less than a 15 HP difference between the engines.
Because of the build quality of the new 6, the motor is damn near eerily quiet. In "normal" every day driving, you WILL be short-shifting it, because you can't really hear it or feel it. It starts to growl at about 3500-4000 RPM with a redline near about 7500.
Unfortunately, the V-6 is only available as an automatic. But I do believe on the sTouring and sGrand Touring you get paddle shifters.
Price wise, the 6 with the 3.7 and loaded to the gills (sGrand Touring with Nav system) will run a bit over $33,000. The sTouring can be had for $3000 less.
The RX-8 Grand Touring, with the same stuff will run a touch over $35,000.
For my lifestyle and needs, as much fun as the RX-8 would likely be, I'd go with the 6.
Plus, the new V-6 only has right at 40 MORE horsepower than the Rotary.
Add that both the I4 and the V6 don't require premium grade fuel, and get better gas milage.