They work. They are not quieter than a standard muffler, but the can's exhaust stinger may be tailored to drive the exhaust note above the frequencies most dB meters are weighted for, so a noise restriction may be avoided.

Here's my method of constructing one:

Mousse Can Mufflers got their start in Unlimited Competition Fun Fly, and accomplish two things.
1. They are lighter than either a stock muffler or a tuned pipe.
2. They provide a pipe-like power boost, but without the peak in the power curve. This was very important to a Fun Flyer when even a one click change in throttle made a large difference and you didn't want all that power coming on at once.

The size of the can is relatively unimportant. If fact, nothing about these cans is critical. I'm using a White Rain (about 1.5" x 4") for my OS.32. An FDS can works well, too, but you might get some funny looks from the sales clerk. A .40 size engine will probably take a standard mousse can (about 1.5" x 6"). Also purchase the header for your engine and a 12" length of brass tubing (about 1/4" for the small engine, and 5/16-3/8" or so for the .40's). I know this sounds small, but you're creating a pressure canister that will boost the mix like a pipe would. You will also need a package of JB Weld, available at Wal-Mart. Now for the assembly:

1. Remove the nozzle of the can and release all the pressure. You might want to do this outside.
2. Using a drill and a sanding drum on a Dremel, open the nozzle end of the can to just slightly under your header diameter. Use needle nose pliers to crimp the edges of the hole outward so the fit on the header is tight.
3. In the center of the other end, drill a tight hole to fit the brass tubing you chose.
4. Scuff the can, header, and tube roughly with sandpaper to create a good surface to glue to.
5. Cut about 1"-1.5" of brass tubing and insert it into the hole about 1/4"-1/2".
6. Mix the JB Weld and apply liberally around the tube/can joint. Let stand for 24 hours, no less, and DON'T move it during that time.
7. Slide the header about 1/2" into the other end and apply the JB Weld. Make sure you get a good fillet here. Let dry following the above instructions.
8. Determine what quadrant of your can will be straight down when the engine is mounted, and drill a 1/16" hole at the rear edge for drainage.
9. Support the can with a fuselage pipe mount as you normally would a pipe.

That's it, sounds hard, but is really easy, and I think you'll like the results.

Dr.1