Pros: airy, fast, detailed, pin-sharp imaging, full-bodied, seductive midrange
Cons: sensitive to...everything, some may find it too warm, lots of power needed for bass control
Some love it, some don't but there is little doubt that this is one of the top-tier headphones out there and one that is not so rare, so if bought second-hand the price of the headphones and amp are very much comparable to a top-level dynamic/planar setup which makes these a compelling option.
The fit is a little tricky at first, but once it is set up it feels very comfortable on the head. It is important to position the transducers close to the ears. The build quality is pretty solid and the finish looks great apart form the cable entry which is prone to break if one is not careful.
Technically it carries all the aspects e'stats are famous for. So it is very airy and presents masses of detail but in a fairly unobtrusive way. It is quite sensitive to what components are in the signal chain and I would say, the simpler it is, the better.
The O2 Mk1 definitely has a distinctive sound that some may call "warm". That is, it injects life and emotion into the music even when it is not recorded that way and adds a bit of extra weight to the instruments. It has that creamy, smooth type of presentation which is easy on the ears so which makes it more forgiving, apart from a treble that is slightly uneven and can get "caught out" on some "harder, more digital" recordings.
The bass has a good amount of texture and weight and fairly punchy by electrostatic standard but could be more controlled.
Because of its own sound character and power need to control the bass, a linear powerful solid state amp should be a perfect match, however if some would like an even more "old-school" tubey sound they might like it with a SRM-007t or other hybrid/all-tube amps as well. The SRM-717 is somewhere in the middle carrying the solid-state refinement but also adding a little extra warmth to the O2 Mk1's generally warm sound and makes the soundstage slightly diffused and it does not have full control over the bass.
So how could I sum this one up?
Well, I think the SR-007 Mk1 is definitely "The Dark Knight of electrostats"
Just listen to the main theme with these, and you will hear what I meant :-) So if you happen to like that sort of thing, you will not be disappointed.
(Main test setup was: Lavry DA11->SRM-717->SR-007)