I own one and can tell you it's quite reliable, as is JMT himself. It's a bit harsh on the upper frequencies for my taste but still listenable, ridiculously portable, inexpensive and economical to run (the amp, not JMT). The single 9v battery it requires lasts for ages, which is due to the op amp (which you might want to discuss with JMT who, so far as I know, does not require a battery). Also: You'll want to tell JMT if you're going to use high impedence phones like the Sennheisers you've mentioned.
People say the cmoy works best with Grados, which don't happen to be among my favorite headphones. The mint-cased version looks really George, but I don't recommend it if you're inclined to carry headphone amps around. (How long does a tin of mints last in your pocket?) If you intend on leaving home with your cmoy, I suggest asking JMT to use the more durable (Serpac) case.