You can check them out at this website. They're kind of expensive. I think they retail for around $300 but you can find them cheaper online. They also make them for pianos and other instruments and you can sometimes find piano Schertlers on Ebay for less than $100, used. I'm not sure how it works, but I believe the concept is different from either a microphone or a piezo.
http://www.schertler.com/prodotti_ing/pickups/guitar_dyn-g.htmI bought mine in the US through the Internet from Guitar Adoptions and got a pretty good deal for the Schertler and an Ultrasound Acoustic Amp.
http://www.guitaradoptions.com/prodcat/customer/home.phpI play nylon string jazz fingerstyle guitar and the Schertler gives a more accurate natural acoustic sound to my guitar than anything else I've tried, either microphone or pick-up wise. I also have an old archtop steel string jazz guitar and I've tried the Schertler on that and the sound is gorgeous. The only problem is that you can't really crank up too loud for some reason, as the sound gets muddy at high volume. But if you're an acoustic player playing to a small room, you can get a wonderful sound from the Schertler, as though you were just playing a loud unamplified guitar. The thing I like about the Schertler, apart from its sound, is that it isn't implanted into the guitar. It's a small round disk about the size of a 50 cent piece that attaches to the guitar's body with putty. The vaccuum is very strong and it holds quite well and you can experiment with it, moving it around the soundboard until you get the tone you like. You can also switch it from one guitar to another.
I've also heard good things about a piezo system by K&K. I heard a guy playing an acoustic jazz guitar with a K&K system in it and I was pretty impressed. Their system uses several piezos that are attached at different places beneath the soundboard and I think their piezos are of better quality than Fishman's, which I have owned and disliked.
http://www.kksound.com/