I own the earlier version of the AirHead portable amp. Different packaging, and uses 2 AA batteries. I purchased it used after I had my Benchmark HPA2 stolen. The HPA2 is a headphone amp on a little PC board mounted to a 1/4" jack, and I believe it is now out of production. I had mounted it in a small box with a DC/DC converter to provide the required supply voltages from a 9V battery. So, technically, it wasn't a "portable" amp, but I made it into one.
I mention all of that because this might not be a fair comparison. The HPA2 was simply a glorious sounding headphone amp, and I miss it still. I got the TotalAirHead to replace its loss. While the TAH is nice, it doesn't come close to the HPA2. Even with the rather interesting "processor" switched in (designed to get the sound out of the inside of your head), it has lower impact, less dynamics, and less bass than the HPA2.
I still use the TAH occasionally, but frankly, the concept of a portable headphone amp is just too cumbersome. My portable use is either iPhone or iPod, and usually stuffed in a pocket. Another box is just a pain, and though an audible improvement, the convenience isn't there. However, I do use an amp when stationary using the same sources. My current favorite is a custom amp I made, and it does make the TAH pale by comparison.
So, this is a luke-warm review. Perhaps the current version is better, hopefully worth the hassle of porting it around. One feature I wish I had on the TAH is auto shut off with silence. I almost always forget and leave it on to eat its batteries.