The MMGs aren't cone speakers, they're planar speakers. The technology is described here:

http://www.magnepan.com/maggie_tech.php

Instead of a cone, the speakers use a very thin ribbon suspended between bar magnets. I'm not terribly knowledgeable about it though, I'm sure others can describe it more insightfully.

I know what people mean when they say the MMGs aren't as good for rock. I've been told they don't push air the same way cones do, so they lack that "punch in the chest" that cone speakers can give you. To some extent this is solved with the addition of a subwoofer, as I have done.

But there are also subjective taste differences; some people might like them just fine for rock (as Florian does), and who's to say they're wrong? If you can find a Magnepan dealer around you, you can try them out yourself.

I will say (and I think nobody would dispute) that for clarity, accuracy and resolution, the MMGs are very hard to beat for the price range. I listen to a lot of ambient/electronica, which tends to be heavy on the production side: complex, multi-layered and tonally/timbrally diverse - for this purpose, these speakers absolutely shine. The level of detail is just spectacular.

They also do very well with jazz - A well-recorded piano, acoustic bass, a woman's voice, a tightly tuned drum kit, and you're there.

Some people may say they lack the dynamic range for demanding classical recordings, but I can't really speak to that.

In my opinion, the speakers give a certain "texture" to the sound. It's hard to explain, but it's sort of velvety; it just feels good in my ears.