The E-M5 is quite a joy to use - there may be fewer buttons, but Olympus has redesigned the menu system to be a lot easier to use and when shooting, you can bring up a quick menu of important settings, which allows you to change things like autofocus, ISO and drive mode, amongst others.
Everything was intuitive enough that I never had a problem figuring out where most of the settings were.
At 16-megapixels, the E-M5 is the highest resolution Micro Four Thirds camera Olympus currently produces. Image quality is very good at low ISO settings and the metering was spot-on most of the time.
Despite the bump up in megapixels, a new hardware and software level image processing engine has seen improvements in ISO performance as well.
In my shooting tests, I found the E-M5 to have about a one stop advantage in high ISO performance over the E-P3.
That means low to negligible noise from ISO 200 to ISO 800, with still usable results at ISO 1600.
It's only at ISO 3200 that high ISO noise reduction kicks in harder and there is noticeable loss of detail, although even here it's still usable if you need to get that picture no matter what.
It's only from ISO 6400 and upwards that noise and loss of detail start to become a problem.
In terms of battery life, the E-M5 performed admirably - in the two weeks or so that I used it off and on, I only charged it once. Even if you are a heavy user, a single battery charge should last you for an entire photoshoot.