I have attended the ATP at the O2 three times in the past and both times loved it!
I went to the evening sessions - once in the very top of upper seats, once in the lower bowl seats near the court, and once was lucky enough to be invited to a suite with work. The upper seats are ok but at the very back in Upper Bowl 3, the court does feel a little far away - I think Upper Bowl 1 & 2 would be preferable. I really loved the Lower Bowl, more so than the suite as you feel right in the action.
It's a good, lively and fun atmosphere at the event - it's intentionally less formal than some of the majors which makes a good contrast. The format is also good, that you can go along for a few hours and see some great games without having commit to a full day.
Access to the O2 is incredibly easy - we took the Jubilee line and it was a little busy but no problems. I usually grab a quick bite to eat afterwards in one of the O2 restaurants before heading home and the train was like normal - you wouldn't know a big event had been on.
I would recommend this event to anyone, whether a big tennis fan or not, I've brought along friends and family who don't watch it normally and they always enjoyed it. The top tickets are pricey but you can normally pick up a reasonable price for earlier days and higher seats.