I don't really like to comment much on threads that compare IQ between sensors or camera coz I don't think I am qualified and most of the time I have been pretty satisfied with most of the current cameras. But I want to share my experience with these cameras because they are indeed different in almost every way, from controls, characteristics, rendering etc...
I shot the OM-D for a little more than 4 months, E-PM2 for a month, E-P5 for a couple of weeks and I had just started shooting with the X-E1 a couple of days ago. Among the three Oly, the E-P5 is my favorite.
From initial impression, there are two things I noticed sensor wise:
1. The shadow on X-E1 files are cleaner at all range (IMHO)
2. The highlights on X-E1 is not as easily recoverable as the sensors in those three Oly cameras
When I shot with the
Oly's I really liked the output I got except when doing long exposures or when I end up underexposing a particular shot a bit, the shadows are not as forgiving and I always ended up with a photo that's a bit too noisy than I would have wanted. Even if I shot the photo at ISO 200 there is something in the shadows. It's just not as refined. I came from a D700 before I shot Oly... so I might have been just my perception.
Now I bought the X-E1 coz I got curious about Fuji,
I was not expecting nor hoping it would have better IQ BUT after a few days of shooting with the X-E1, my impresions are:
-The controls are very nice
-AF is a lot slower than any of the above Oly cameras
-E-VF is worst than on the E-M5 or VF-4 and while it is better than the VF-4 color wise it lags a lot (I knew this before I bought it so it's okay).
But the files I get from it seems
smoother, cleaner, more refined in almost all ISO.
Also, like Gary said it forces you to shoot slower, compose better, think before you click the shutter, inspires you more. For everyday use I can see myself using the X-E1 more than any of the Oly for the foreseeable future.
I am not sure I got better photos out of the X-E1 but the shooting experience is definitely better. Also I will definitely get one Oly body again in the next few months as my action camera... (most likely another E-P5).
Sorry for the long post.