I note that the person calling the Mazda 3 a POS is driving an LS400.
Of course it's going to seem cheap and noisy to someone driving a big Lexus.
I also recently purchased a Mazdaspeed 3 after driving four different Miatas over the last ten years and, coming from the other end of the spectrum, I'm loving my MS3.
I really fell in love after my first long highway trip in it. I drove highways non-stop from Galveston to Austin and the low end power is so much better that I didn't have to take it out of 6th gear until I got of the highway back in Austin, and the ride and noise levels are so much nicer than I'm used to. I had two friends in the car and we were talking the whole time at normal volumes -- something I couldn't do in the Miata. It seems downright plush to me.
It's all perspective, but it's by no means a cheaply built car. As others have mentioned, it's been getting some great reviews as of late from numerous publications and there was a stock engined MS3 that recently placed 10th overall in a 2000 km rally in Australia. Betcha can't do that in your LS400!
There are several aftermarket manufacturers that have recently introduced stuff and/or are working on stuff for the MS3. If you want places to look, PM me and I'll pass along som URL's.
CAI's are adding about 25 HP and 25 Ft Lbs. and there's a piggyback ECU for a grand that adds around 60 HP and 70 ft lbs with the tuning that it allows. Easy 300 HP at the wheels without opening the engine or touching the turbo, intercooler or exhaust.