I purchased one of these from Smartune - Dan even came up and installed it for me at my house. I thought I'd post some first impressions for everyone!
The first thing I noticed was before Dan was even done. The OEM muffler is HUGE. OMG that thing is overkill... I swear it looks as big as the muffler on my Chevy Tahoe. Dan says the OEM unit weighs 23 pounds while the Doug Thorley unit weighs 6. Woo-hoo! I can carry 17 more lbs of groceries now!
So the big question everyone asks... is it noisy? In a word, yes. But it's no louder than the wind noise on the highway, or even the road noise on the highway for that matter. It's a new sound being added to the cacaphony, though, so you'll absolutely notice it.
Under normal "smart style" acceleration it sounds like a big cat purring (meaning tiger, not big kitty). There's a deep-throated resonance to it that I kind of like. The car revs up and shifts as always, but now it has a soundtrack. While tooling along the highway simply maintaining speed the sound can fade until it is barely audible. If you're fighting wind, though, it gets a little louder to the point that it might interfere with your enjoyment of the radio depending on how loud you have it cranked. (I typically keep my radio between 8 and 11.)
If you stomp on the gas pedal, the car growls and snarls. If you use manual to force the revs up, it gets pretty loud and sounds like a superbike howl. Only not as fast. ;)
For comparison, it's not as loud as most motorcycles, especially Harleys. It's also not as loud as older diesel pickups. But it is louder than just about any unmodified gas car.
On the power side of things, I can definitely tell a difference. It's not a mash-you-into-your-seat difference, but the car does accelerate more quickly now.
Oddly enough, the exhaust seems to have solved two nagging issues I used to have with the transmission. The car now holds 2nd gear slightly longer than before when starting from stop, which means I can now clear intersections before shifting twice. That used to always make me feel like I was in the way because those first two shifts would happen right in the intersection. Second, the car now chooses to upshift into 5th as I'm going down hills instead of holding 4th all the way down. I never could figure out that behavior before, but now it's gone so I'm happy.
I haven't run a full tank with the new muffler yet so I can't give a definitive answer regarding gas mileage. However, I have been keeping detailed mileage notes ever since I've owned the car and this tank looks like it's going to be my best mileage to date. I will probably top 45 mpg for the first time ever, and if it keeps up at its current pace I may even beat 46 on this tank. My car has been averaging 43-44 mpg through 6 tanks and 2100 miles.
One further note on the sound... I suspect that the new soundtrack does as much to help your mileage as the muffler itself. Why? Because once this exhaust is installed you have an audible clue every time you're on the gas. It's almost like the car is growling at you telling you to slow down! It also serves as cruise control if you have a good ear.
If Dan does your install for you, he'll take you for a quick ride in his car so that you can get hear the difference before you install.
I'm still very happy with the exhaust, though, because the power is noticable and I'm really growing to like the sound. 
Today
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