Pros: Very accurately reproduces the source signal; I guess it could help if the source's headphone jack isn't strong enough
Cons: Could use more power
The objective facts
Background: I listen to music on a higher-end (purchased for $800) Asus laptop, or my Galaxy Note 2 smartphone. With my SRH940s, the phone sounds better in terms of sound signature, but with my Sennheiser HD800s, the laptop sounds better for some reason. But they're both very close. All of these experiments were done listening to music on Spotify 320kbps; that's how I listen to headphones 99% of the time so that's the only way to test for my purposes.
How I tested this: I connected the O2 amp, with low gain, to my laptop's output jack. I set the volume of the O2 amp to its highest level (in other words, I controlled the volume of the music from my laptop's volume meter). The playback was set at 24bit / 192 khz (basic windows audio lets you pick this option). I plugged the HD800 into the amplifier.
The first thing I noticed was that this setup didn't cause the HD800 to be that much louder than it was when I simply plugged it directly into the laptop. In other words, with the O2 amp's volume set at max, and my laptop volume set at 25%, the headphones had close to the same volume as when I plugged the HD800 directly into my laptop and set the volume to 25%. So my laptop has as much power as this thing. Then I turned up the volume to the maximum and used a decibel meter to see how loud the two could make the headphones, and again, very similar. I repeated the above with the phone and got more or less similar results.
Next I cycled through a few songs that are very well recorded, to see if they sound better on one vs. the other. I listened once, twice, three times, again, again, focusing on very specific parts of the song, often playing 10 second portions of a song and repeatedly alternating between the two sources. I could find nothing. I would often focus on one tiny detail in a song, to see if the O2 and computer produce it the same, and they could. When the O2 was driving the phone, it had the phone's less desirable sound signature, and when it was driving the laptop, it had the laptop's sound signature, but the O2 didn't change the sound at all. I thought an amp was supposed to improve the sound of my "power hungry" HD800, but this did nothing. However, commenters say that this is exactly how it's supposed to work. In other words, it's not supposed to change the sound at all. So I guess I can't rate it poorly for doing what it's supposed to do.
Conclusion: If you have a severely underpowered source and need power, this will deliver it. But it could do more in this respect. It's not underpowered like some of those pathetic portable amps you see out there. But in my opinion, a $130 desktop headphone amp should provide boundless power, as much as you could ever want. When you buy a desktop amp, you should no longer be concerned about not having enough power. You should be concerned about keeping the volume low so as not to blow out your ears. To provide technical stats, on its website, it states that the O2 can only deliver a maximum of 88mW at 600 ohms. Well what if you're trying to drive a T1 or another 600 ohm headphone? Is 0.09 of a watt enough? Again, it has power, but you might not need the little bit of extra power this provides, and you might be fine sticking with your laptop or phone's amplifier. In light of the fact that it costs $130 and sits on your desktop, but doesn't have huge bountiful power, I'll subtract 2 stars. I'm just not sure why you would buy this if you want a desktop amp.