They take perfectly good mixes and add the ability to go into self-oscillation at the drop of a hat...
They boost the frequency ranges that you usually DO NOT want to boost on live mics. Then they spin the phase relationships around for good measure. Finally, they remove a good portion of the sound systems' built-in headroom so that those pesky peaks in the dynamic range clip the power amps at the extreme ends of the response curve to create nasty, grating harmonics that a dog can hear at 500 yards.
I have used them to restore some fidelity to playback of cassettes at times, but who uses THOSE anymore? Save your money and buy good mics that don't make you yurn for "more tone".

LOL- Nice Jim totally enjoyed that!

To think I was gonna respond with the classic "They maximise your sound".

I am sure to receive more emails from manufacturers for bashing product, but hey, if I can help a brother out, then....

They are a waste of money. There is no justifiable need for them, EVER. Do your homework. Design a system that will cover the majority of your venues. Choose components that will have synergy, like guitars, pick-ups, strings, amps and players have synergy. There is more to it than meets the eye. Less gear in the chain on clean power is ALWAYS better.

And I've always seen the BBE Sonic Maximizers. I was just wondering what exactly do they do?

Meh. I suppose they work well for crappy equipment or recording direct, but other than that I see no benefits from sonic maximizers.

i have to pipe in here.

i haven't performed in 5 years but when i was gigging regulalry i always used a BBE462 on my vocal mic. I am a pretty good singer, so i wasnt using it to hide anything, but i felt that it always tweaked my signal quite nicely.

although i would never endorse a SM for recording, it do believe they have a niche in a live situation.

Besides, if you are musicians and not sound enmgineers, they are so cheap it wouldnt hurt to try one.

Because they are cheap is THE reason not to try one. Less techno crap in the signal chain = greater signal to noise, dynamic range, less smearing, etc.

sm57's are cheap too :-)

SM-57's are not "cheap" in the way Sheet referred to the Sonic Bastardizer. They are inexpensive and a helluva value these days.
I stand by my statement, coming from 30 years of fighting feedback and power amp clipping. You place a SM, AE, or any box with a few chips fueled by a wall wart into the mix bus to "extend" the frequency response of a system, and you be lookin' at big problems. Headroom is not where you store the groupies...

"Headroom is not where you store the groupies..."
then it's the bathroom on a ship, right?

they're good for program music (ie- a cd or ipod), but poor for a live band.

they essentially add odd- and even-order harmonics... like an aural exciter.