It's quieter inside. Everything on the outside seems more muffled. Trucks right next to the car don't sound that much different. But airplanes overhead and outside sounds in general are quieter now. My wife can't tell when she calls me in the car if I'm in the car or still sitting at my desk.
The huge difference in sound is when it rains. I only hear rain on the windows now. I thought I would still hear rain through the roof but that it'd be damped. I hear nothing though. No more of that pinging, huge resonant sheet metal sound. It sounds like a good speaker box when you rap your knuckles on it now. Dead. Solid. You'll see on the top of the headliner are lots of plastic cubes that look like they contact the roof. This probably transmits a lot of noise. Some of the noise reduction may come from isolating these plastic bits from the roof with the 3M material.
For removing the headliner, I popped the a-pillar trim panels off. They're tethered to the car but you can remove them by twisting the plastic cable. Then I removed the b-pillars. You have to remove the lower portion first by popping off with a trim panel remover or something similar. The top portion then unscrews and drops down. The c-pillars/sail panels are held on by one screw each behind that access panel that says "airbag."
Then take the assist handles off the rear sides. Just two screws each. The visors are held up by a screw on the side that swivels. Remove that and unplug the wire that powers the vanity lights. The clip is held on buy a sliding peg type of thing. If you look at the front, there's a groove you can get a pocket knife or small screwdriver tip into. Slide that peg out towards the windshield and the clip drops.
At this point the only things holding the headliner up are two sets of clips on either side of the rear view mirror and back by the third brake light. I pulled hard on one side then the other and it came down pretty easily. Be careful at the back because it's pretty easy to break the clip that holds the third brake light's wire to the light. Mine goes back in, but it's not secure and comes out sometimes. Have to figure out how to secure the wire so it stays plugged in.
Applying the tiles from Sound Deadener Showdown was easy. They're precut to 6"x10" and they seem more substantial than Dynamat Xtreme. I used 11 tiles to deaden the whole roof. Just spread them out and use a wooden wheel to mash them down. I tried to keep the distribution pretty symmetrical side to side. Rap the outside of the roof with your knuckles to see where may need more coverage. 25% coverage was fine for me.
The 3M stuff I cut in half so I would up with two sections that span the roof from side to side. It worked out that one section did from the front of the roof back to the b-pillar and the other section did from the b-pillar to the back of the roof. I had to trim quite a bit of the 3M so the sides, front and back of the headliner would fit back up. Probably 4-5" off the perimeter. Cutting and tearing this Thinsulate puts a lot of little cotton-like fibers all over.
The real pain came with getting the headliner secured back up. It's just because there's fluffy blanket now between the headliner and roof. It's a lot tighter. I started with the assist handles in the back. I tried to push the clips by the 3rd brake light in first, but they couldn't hold the weight of the whole headliner plus the 3M blanket. Take your time and maybe trim around the pieces that have to be screwed in, like the assist handles and the visors.
Worst part was getting the visors back up. Very tight and I cut the insulation on the vanity mirror wire on the passenger side trying to put it back up through the metal. I left it unplugged then fixed later on. The clips that hold the side of the visors that swings out were hard. Take them apart so you can see how they work. The little square peg slides out and there's a spring with a plastic piece under it. That plastic piece is the round part you see on the outside of the clip that holds the visor secure. There's a small square section of the roof that protrudes into this assembly and the spring actually grips a little hole cut in that square. You have to push the clip up against that square section as hard as you can. The square peg then slides in and it has to go on top of the sheet metal square. There's a notch in the back of the clip (relative to the car) that this peg has to sit in when you push it in. I didn't take any pictures of this assembly and I'm not taking it apart again to take any.
I wound up having to enlarge the hole through the headliner to get this to work but you can't tell. It sucks because I thought I got it in right but then the peg hadn't seated flat against the sheet metal and the little spring loaded retainer that holds the visor in wasn't secure. So you'd push up against it and there's no resistance on the spring. Start over. I attempted it probably 100 times before I finally got it locked down.