-- Size: very small and pairs well with a J3, iTouch, Sony X-Series, iPod Classic or similar device.
-- Battery life: as said before, simply amazing!
-- Construction: very solid and well built. Great switches, with a firm feel and nice "click" to them
-- Form Factor: Second to NONE! All of the inputs, outputs and switches are perfectly placed.
-- Weight: surprisingly very light. This kind of caught me off guard.
-- Sound: The good stuff...
1. Bass: Great (setting "2" can be a little bloated on bass-heavy tracks)
2. Mids: Realistic and clear
3. Highs: Also realistic without a hint of shrill or sibilance
4. Gain: I primarily use setting "1" for just about everything and setting "2" for my HD600s. I have no need for setting "3" (which is probably designed for 600 Ohm headphones). This thing has plenty of oomph.
5. Impedance: Aside from when I'm wearing my Grados I keep this setting on "0". It tends to veil the highs too much for my taste. I only occasionally put it on setting "1" when wearing my SR225i's.
6. Crossfeed: The best implementation of Crossfeed that I have heard so far. Setting "1" is perfect for long listening sessions when you want to minimize fatigue. Setting "2" is great for hard-panned music. Both settings do a great job without significantly altering the sound quality.
-- The volume pot: there is a slight imbalance in the first 10% of the range, but this is common on almost all non-digital volume controls. Heck, even my NuForce Icon HDP suffers from this.
-- Impedance: this setting is effectively useless because it veils all frequencies above 4,000Hz so much that it isn't worth using (sans Grados).
-- Hiss: With about 10 different headphones in my collection only 3 of them experienced hiss (and subtle at that, so I am being very nitpicky here). The three that showed a subtle amount of hiss were the UE SuperFi 4, Denon AH-C710 and Sennheiser MXL-570; three headphones that really shouldn't be amped anyway so it's a moot point. The others have enough resistance to negate any hiss.
Overall, this amp is a 9 out of 10. Right now, I can't think of a better portable amp on the market.