I have a 2007 Mazdaspeed 6 base model in Black Mica with an add-on Mazdaspeed CAI.
LIKE:
6 speed shifts really smooth compared to several other manual transmission cars I've driven.
I love how stealthy the vehicle is. No hood scoop, barely a lip on the tail, nothing that screams "ricer" or "pull me over."
AWD with snow tires are nice for our Wisconsin Winters.
Roomier than most other "similar" vehicles (turbo-4, AWD, sedan) except for the Legacy.
Great cornering ability.
Great gas mileage. I average around 23-24MPG driving like I stole it. Sustaining around 80MPH on long trips gets around 25MPG. I've gotten 30MPG on long trips with cruise control set to 65MPH.
Seat shape fits me well.
I rather like the factory stereo headunit (6CD MP3). Works really well.
DISLIKE:
Clutch can be touchy. I've been driving manual transmission vehicles for decades, and when I first got this car I was stalling all the time from a stop.
Lousy acceleration from a stop compared to similar cars.
Stiff suspension can make for rough ride.
No folding rear seats due to extra structural support in the Speed version of the 6.
Seat cushion a bit hard.
Factory Bose free-air subwoofer in the deck puts out almost zero noise. Not a problem with my unit, but with the design or factory equalization because other ones at the dealer sounded just as lousy. At least it's better than the Mazdaspeed 3 factory subwoofer, which is a 3" (yes three inch) driver in a tiny box under the passenger seat. Most people don't know that it exists since it is not capable of making itself heard or felt.
I wish the factory stereo had some kind of standard AUX input, or USB, or whatever. Only option was for an iPod connector. No, I'm not going to buy an iPod just to use it in my car.
PROBLEMS I'VE EXPERIENCED:
I've gotten a lot of CELs, all from two sources. One comes back as the fuel filler door not sealed, which is BS. Had it a few times in a few month period and then problem went away. Another is some intake code. I've been getting that lately after my car gets wet such as driving through puddles or getting the car washed. It might be that I need a new air filter - I have a CAI that has the bare filter just in front of the driver wheel well and sitting really low. Didn't start doing this until recent months.
The factory Bose subwoofer now pops a lot, but I think it is because I fiddled with it using someone's homemade EQ/booster trying to boost the output.

There's a "rattle" that I seem to feel through the steering which developed this past year.
OTHER THOUGHTS:
In retrospect maybe I should have gotten the higher model to get the nicer interior.
I keep having the feeling that it would have much better performance if it were lighter in weight.
I wish the Cherry Mica color was available for the 2007 model (it was only 2006) because it looks HAWT!
I wish the factory speakers were better. Bose is so... meh. Sounds decent, but not that spectacular.