I may be late to the party on this one, but I'm just starting to discover mid-side EQs and a whole new world of processing is opening up. Thought I'd share!
Quick intro: a stereo field is created by small differences between the sound coming out of the left and right speakers. With a mono signal, all the information between both speakers is exactly the same. With a stereo signal, some of the information between both speakers is different, and some of it is (usually) the same.
What a mid-side EQ does is allows you to treat the mono component of a stereo signal separately from the stereo component of the stereo signal. A great way to use this is on drums - take your mid/side EQ and slap it on your kick. On the side part of the sound (the stereo component), add a high pass filter at about 100 Hz - your kick's low end is now in mono! What this is doing is simply filtering out the low end of the stereo component of your kick. This way you can keep your kick's nice, stereo top end but still have the low end punch that comes out of having your bass in mono.
If you don't need to do full-on multiband processing and simply want one part of the sound in mono, this is a great way to quickly do it. You can really change your stereo image this way, its awesome.
If you are a Logic or Live or ProTools user you should definitely check this out! All of them have built in mid-side EQs.
