Like the title says, this CMoy from JDS is killer! Great sound in a small package for less than half of what many comparable amps cost. It adds a nice warmth and depth to the XBA 3s and GR07s I've tried. The 9V easily powers most headphones, even those that are harder to drive. The bass boost is rarely needed, but if you want some extra bump, the CMoy has plenty to go around.
 
Like many portable amps I was wary about the battery life, especially for something so inexpensive. No worries anymore. I has been playing since mid-morning and it is now 6:45. Also, the rechargeable battery will allow it to play while its charging - a serious step up over Fiio's amps. 
 
The volume knob is very sensitive and the amp will turn off automatically if the headphone cable is disconnected. Great feature!
 
The only things I can imagine would bug some people (who shouldn't have ANY problems in this price range...) is that the Altoids tin is a little funny (although unmarked tins can be purchased). The lack of gain or impedance control has been mentioned but honestly for everything else you get, that's like missing the sprinkles, on the icing, on the cake.
 
Thank you Jon for a great product!

Out of the box, these guys are impressive. Killer looks, minimal packaging, obvious sturdy construction - in other words, German. 
 
The sound was very good. They're easier to drive than most Beyer over-ears and they have a very good sound. I wouldn't say great. Coming out of my iPod Classic, the wider-than-average soundstage provided some excellent space for my live rock recordings. The bass was solid and got even better with my Fiio E5 - I imagine a heftier amp would do even better. The mids are so-so, not recessed in my ears but definitely not forward. The trebles were lacking, but they probably aren't supposed to be audiophile quality for the market these are targeted at anyway.
 
Overall I like the COPs, although the ~$200 price tag may be hard to justify for some. I got a great deal so they were worth it. I think a comparable sound for cheaper would be the M50s.
 
Hope this helps!

First impression? Wow. 
 
So glad I joined Head-Fi! If I hadn't I don't know if I would have been turned on to these classic phones. Great sound, lifetime warranty, great comfort, oh and did I mention the no-questions asked warranty?! 
 
For the record, I've got the KTC model (for iThings) and the controls and sound for calls is right inline with the rest of the product. What a wonderful pair. Thank you Mr. Koss!