Hi Philipp,
I have an OM-D E-M1. Yes, currently it is the flagship of the Olympus line. I haven’t shot with the E-M5 so I can’t make comparisons between the two. I do have experience with Canon full frames and APC-S cameras. The E-M1 produces excellent images but the sensor is nosier than my FF. This is overcome with Low ISO and slightly overexposing the image (1/3 to 2/3 of a stop) and then knocking it down in post. The camera doesn’t have an AA filter and has a pixel density of 7.24 MP/cm2. Camera is capable of capturing some eye-popping detail. It is the equivalent of a 64MP Full Frame sensor.
Enough about the specs, ergonomics and handling – absolute joy to use - very lightweight and rugged. EV is crisp, clean and has very good resolution as does the back panel LCD. I don’t notice a lag when using EVF. I like the function and position of the two spin wheels and flip lever which gives the photographer quick access to SS, ISO, Aperture, and WB. Camera is highly customizable and you can set your own functions. It’s a very well built camera, weather-sealed and rugged which makes for a robust field camera if you like shooting nature and such. AF is very good for a mirrorless camera. I must note I only have one native u4/3 lens so my experience with AF has been limited.
There are some very good u4/3 lens out there. The format and mount isn’t limited to Olympus - Panosonic and Voigtlander come to mind and there’s a sprinkling of a few other third party lens. Steve Huff has a good review on recommended lens for micro u4/3:
http://www.stevehuffphoto.com/2013/08/18/the-best-micro-43-lenses-reviewed-in-case-you-missed-them/
Absent from Steve’s list is the 40mm-150mm f/2.8, recently released. By all accounts this is an excellent lens and worth consideration if you are interested in the focal lengths. Real benefit of a u4/3 camera, for me, is with an adapter one can use just vintage lenses from all manufacturers.
If you are considering the EM-1 you may want to wait as the E-M5 MKII will be released in the next few months and prices on the E-M1 will drop. Personally, I really like the camera, and have been so impressed with the performance I have been mulling over the idea of selling my Canon gear but I'm just not at that point, yet…..