Pros: Extremely low noise floor, wire-with-gain sound aka you´ll notice new details in songs, made in England, solid build quality
Cons: None at this price, except that it might make you feel bad about some previous purchases, gain even on 1x mode is too high for some sensitive IEMs
What can I say... The amplifier designed by The One Who Cannot Be Named, produced in mass by a new British audio company. I´m not going to cover the design philosophy of the amp as anyone can Google the O2 amplifier page to find out what it´s about. What sums up everything I can say about this particular amp is that I used to own a Benchmark DAC-1 PRE and I´m pretty confident I couldn´t tell it apart from this in a blind test :)
If you are looking for an amplifier with intentional coloration/pleasing euphonic sound, look elsewhere though. I don´t think the O2 succeeds in making all intentionally colored amps obsolete - there´s a space for some of them, but for most people I think it just makes more sense to get a neutral source like this and concentrate on buying different kinds of headphones instead. For people new to Head-Fi, you can now get an amazing budget rig by getting this and the EDAC and a good pair of headphones like the Sennheiser HD 600/650. For many it´ll be a very nice end game rig. On the other hand if you´re a more experienced headphone hobbyist there´s no reason not to check out the O2 amplifier (even out of sheer curiosity) as it costs next to nothing compared to what we´re used to paying for our amps.