Sounding poor, mostly. Crushed dynamics, prone to clipping etc.. It doesn't offend me, it's just not good. A shame, given all the hype at the time- I was hoping for something that sounded clean and didn't harm the sound. Having heard a few off the shelf pre-built ones, as well as one built by a very competent DIYer, I've probably heard more O2s now than are healthy- due to hoping that the first one wasn't representative.
The Fulla 2 is a lot easier on the ears. It uses nicely-chosen op-amps on the output, the device achieves remarkable efficiency and manage to kick out most of that paltry 500mA that the device is allowed to draw. Don't expect it to sound like a Krell or somesuch, but for something that you can throw in a pocket and costs $99, its pretty slick.
As to the "how", the Fulla 2 doesn't fall into the same category as it has been designed by a skilled and experienced amp designer- Jason Stoddard. He carefully finessed the BOM and power budget to hit a target. It's not his first rodeo, and he knows the tradeoffs. His designs tend to maximise quality at a given pricepoint. He's not perfect, he screws up sometimes, but he's a smart cookie, and his stuff works.
The O2, on the other hand, is a poor design from an inexperienced amp designer, in an attempt to rig a very specific set of measurements with no care given to how the device actually
sounds. That doesn't mean that being objective can't give you solid, decent sounding amps, of course. For example, Tom Christiansen is an almost obsessively objective engineer who has designed
some decent amps that also measure beautifully, but he does so with his eyes open.
Simply, the O2 isn't very good, there are better choices for the price. Simply, if the answer is "an O2 amp", you may wish to rephrase the question. I'd stick with the headphone outs of my phone/laptop/whatever in preference to using an 02. Hell, even a little CMOY would sound better.