In the for what it's worth department (since i am an infrequent poster and do not consider myself any sort of an expert)
I second using these the Leight Impact:
http://www.amazon.com/Howard-Leight-R-01526-Electronic-Earmuff/dp/B001T7QJ9O/ref=sr_1_1?s=sporting-goods&ie=UTF8&qid=1314637716&sr=1-1
They are cut down at the bottom so I don't find they impede the cheek weld, at least for me.
I tested the attenuation and ambient amplification like this: I set the TV volume in the computer room to where I could not hear it in the family room next to it. Put on the muffs, and set the amplification. I could now very clearly hear the TV, and with the stereo reception on the muffs, the sound quality was good. I then cracked a foxhunting whip (the popper on mine sounds like a .38 special going off) It muted it just fine IMO. Not as much as my passive muffs, but I didn't expect it to.
I don't exclusively shotgun, I shoot a lot of .45, .44 mag, .357 mag and 5.7 in handguns (that last is LOUD lol), and in rifle a number of calibers from .223 to 8mm Mauser.
The sound you hear doesn't just get inside your head through the ear canal, it is also transmitted by the bone of your skull and other parts of your head. According to what I have read, an in-canal ear plug does not block this "extra" sound. I can corroborate this finding. Having flown small airplanes for a number of years, I chose the earmuff style head sets over the in-ear canal style because of this very issue, having direct experience rather than just relying on reading. The plugs just didn't cut down the noise enough for my taste. Now this "extra" sound is admittedly much less than the direct canal transmission, but over time it all adds up, and I would like to go into my "older" old age with some hearing left.
If i am shotgunning only, i might use my in-ear canal protection. But if i am spending a full day at the range shooting a variety of noisy toys, or i am shooting at an indoor range, the muffs will always be my choice.
Again, opinion is worth what you paid for it. YMMV Like anything else, test and find what works for you.