I've been using this amp for the last few months to power multiple pairs of headphones that I own. The three main pairs that I use are the Sennheiser HD598, the HD6XX and the Hifiman HE-560. The amp itself seems to have a pretty low noise floor, as even plugging in pretty sensitive IEMs doesn't transmit anymore than a tiny amount of noise, if any.
The 598's do sound a bit more open, and although this isn't a huge difference, it is pretty nice since these are my headphones of choice for playing games, especially positional audio games like CSGO. The bass response of the 598's, which isn't necessarily lacking, definitely sounds stronger when hooked up to the O2.
The 6XX sounds pretty amazing with this amp. Obviously this isn't a headphone you'd typically want to run without an amp anyway, but it pairs very well with the O2. Compared to listening to them through a non-amplified source like a phone, the only real way I could describe it accurately would be to say it feels like the 6XXs almost "open up" as if they were removing a shroud that was covering them previously. Everything sounds more clear, more powerful, and the channel separation seems much better too, although that could be placebo or simply the combination of being able to hear more detail.
The HE-560s are probably the most power hungry headphones I own at the moment, and despite the O2 amp only putting out somewhere between 300 and 600mW at the 45Ω impedance and 90dB efficiency, it sounds amazing. Bass hits hard (as planars should), the treble is clear and sparkly, and mids are warm and enveloping. As much as I'm sure the 560 could take more power from a bigger amp, I have trouble parting with the money to buy one since it already sounds so good off the O2.
Bottom line, this is a phenomenal purchase, especially at <100$, and I'd highly recommend this amp to anyone. I'm actually thinking of getting one to put on my nightstand for use when I don't want to have to sit in my computer chair to enjoy my music!