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I had Flowmaster 50HDs on my truck from the time it was about 3 weeks old until recently, about 5 years total. Pretty impressive sound, not overly loud, but nice. Always had a bit of a drone though and rite at interstate rpms.
Looked to be in good shape from underneath (now keep in mind, the truck has ALWAYS lived between Ga and Fl, so no salty roads). Some surface rust, but nothing major. When I finally removed it I was really surprised to see that the top looked like swiss cheese!

Any independant data I've seen has shown that all but a select few Flowmaster models do indeed flow worse than the stock muffler. Not a real good choice IMO.

Now running a Cherry Bomb Extreme. NO DRONE and it's actually a bit quieter at idle than the FM was, but when given even just a slight bit of throttle it REALLY comes alive. Very aggressive, deep, throaty sound.

Magnaflows consistently get good reviews on Hemi trucks as well, and would be a great choice if you don't want a loud exhaust.

When you choose a muffler, there are basically three types of designs to consider:

1) Glass Pack mufflers use only absorption to reduce the sound. On a muffler like this, the exhaust goes straight through a pipe that is perforated with holes. Surrounding this pipe is a layer of glass insulation that absorbs some of the pressure pulses. These tend to be on the louder end of the scale and because the exhaust gases pass straight thru, are often the best flowing. In the Cherry Bomb lineup, these would be the traditional Glass Packs.

2) Wing Plate Mufflers like the Cherry Bomb Extreme uses a unique design that deflects high frequency sounds off the plate and into the walls of the muffler. Without getting into the laws of physics and sound, these frequencies basically bounce into each other and cancel themselves out. The low frequency sounds vibrate the wing plate and create a very deep exhaust note, in principal, working much the same way a woofer vibrates in a speaker. Because these mufflers are fairly straight thru, with just a sophisticated wing plate, they also tend to flow quite well.

3) Chambered Mufflers are the third type and can be the most complex. Depending on their design, they can have resonating chambers designed to make for a very quiet exhaust, or they can reverberate in much they same way a wing plate does and produce a deep throaty exhaust note. The design of the chambers will dictate how much and what frequency sounds are cancelled out, what sounds are allowed to pass thru and what sounds are enhanced. Depending on design, a chambered muffler can be almost as free flowing as a wing plate muffler or can be downright restrictive as hell. Most mufflers on the market today are a chambered design including the Cherry Bomb Turbo, most Flowmasters and Magnaflows.

Some mufflers incorporate two or more of the above designs, the Cherry Bomb Vortex is a good example, as it has a wing plate, some chambers and it's walls are packed with a woven matt of fiberglass. Although it incorporates chambering, the design flows quite well, actually outflowing most mufflers on the market.

Although it's usually most desirable to have a better flowing muffler, some mufflers are designed to be restrictive in order to create backpressure. Backpressure increases the low end torque produced by an engine. However, this is an inexpensive "band-aid", because backpressure is essential an inefficient loss of power. Most stock vehicles are NOT setup to utilize all available power from the factory and will show low end benefits from backpressure. However, most experts agree that the "right" way to increase low end torque is with proper modifications instead of an intentional increase in backpressure.

A muffler is generally considered to be a "good flowing" design if it's rated CFM is 500 or higher and don't go by the name, check them out for yourself before buying one. A Gibson "Superflow" is rated at a lousy 311 CFM. My old "Flowmaster" 50HD has a 362 rating. My Cherry Bomb Extreme is at 512, a significant improvement. Some VERY HIGH END racing mufflers such as the Dynomax UltraFlow SS can be as high as 1000 cfm. The use of such a high end muffler would, however, significantly decrease performance and could actually result in engine damage without other modifications. For the average stock to mildly modded vehicle a top threshold of 600 should be used. This is incidentally, the CFM of a straight thru 3" pipe with no bends...