Just because a car has "the best" engine doesn't make it the best car. The midrange driver, good as it is, is only one part of the speaker chain, and the implementation of the entire speaker is at least as important as the drivers themselves.
ATC built their company in the professional market and has only recently began making a serious effort to enter the consumer market, which is why we don't hear about them much at this point. They are incredibly good speakers though, so I suspect this will change over time if ATC is able to procure more distribution and trickle their technology down to more earthbound prices. I do agree with Sean that ATC devotees are almost cultlike in their support of the products, much like you see in brands like Linn and Naim. Basically if you get it, you get it, but I've never known anyone to hear a pair of ATCs and not be at least a little impressed.
That said, ATC speakers are still based on a professional monitor platform and thus will not appeal to those who prefer a more "audiophile" sound. This means that you are not going to get an incredibly expansive, deep, or layered soundstage, and ATCs are not going to pull off the disappearing act as well as many others. The flipside is that ATCs are extremely transparent, clear, dynamic, and have an ability to put the power of a performance in the room like few other speakers on the market. Whether this is "the best" is a subjective and personal decision--no speaker is perfect.
Tim
