http://www.luminous-landscape.com/reviews/cameras/olympus_om_d_e_m5_review.shtml
The bottom line on the Olympus OM-D E-M5 (other than its convoluted name) is that it's the best Micro Four Thirds cameras yet, and highly competitive with the current mirrorless segment market leader, the Sony NEX-7. Panasonic will undoubtedly have a strong response product later this year, but they'll have to really up their game if they're going to compete with Olympus in terms of features and functions. No doubt the next Pany will have killer video capability (which is not the O-MD's strong suite), but right now when it comes to fit, finish, features, ruggedness, and all-around camera goodness the O-MD will be a tough act to follow.
The inevitable question will be – "Which is better, the NEX-7 or the OM-D E-M5"? I used the Sony NEX-7 for five months while I was in Mexico last winter, and shot some 6,000 frames with it, so I'm pretty familiar with both its strengths and weakness. I'm still very much familiarizing myself with the new Oly. But after a few weeks and some hundreds of frames I have a pretty good sense of what it can and can't do well, and the "can't" list is very short indeed.
At this point I'd judge there to be little to choose between them in terms of image quality. At 24MP and with an APS-C sized sensor the Sony has an edge in terms of file size, resolution, and shallow DOF. But unless strong cropping or very large prints are the order of the day this isn't an overwhelming advantage. When it comes to overall image quality the O-MD offers very good dynamic range and highly accurate color. I haven't done a side-by-side comparison (my NEX-7 is in for repairs) but based on experience, when it comes to high ISO noise performance these two cameras are very close.
In the end choosing one camera over the other should come down to an individual's personal preferences, and the availability of needed lenses. Right now the MFT camp does have more high quality fast primes than does Sony, something that I value. And, if you add the Olympus Four Thirds lens line, there's no contest in terms of range and quality.
The O-MD is a more traditional design, and offers a very high degree of customizability. The Sony's design is one that eschews the past, sometimes to the good, and sometimes not. This is going to very much be a matter of personal preference, and I would urge you to visit a local retailer and compare the two in-hand. (And while you're there, purchase from that dealer. If you comparison shop locally and then buy on-line all you're doing is hastening the end of the local retail store. Not something to be wished for).
When it comes to lenses the Olympus has a clear edge, being able to call upon not only its own range of MFT lenses, but also Panasonic's. And of course there are all those terrific Olympus high grade Four Thirds lenses as well. With third party and legacy lenses the NEX-7 offers focus peaking, but the O-MD has in-body stabilization, which I regard as a trump card for those interested in working with non-Olympus glass. A stabilized Leica 135mm f/4 APO-Telyt M on the O-MD is something to be experienced.
The only area where the OM-D (and all MFT cameras for the matter) offer any disappointment is their restricted ability to create shallow DOF images, particularly as compared to full frame. APS-C suffers a one EV disadvantage in this regard and MFT two stops. Your call as to how important, or not, this may be for the type of shooting that you do. In this respect I don't regard the 1EV disadvantage of MFT over APS-C to be that significant. Against full frame it's another issue. But the penalty that even the smallest full frame camera extracts in terms of body and lenses size and weight is not to be denied either.
Simply put, the Olympus O-MD E-M5 is a winner, and has now become my preferred camera for travel and urban walk-around shooting. The Nikon D800/e is still my main squeeze – an awesome camera in almost every respect – but, for its price and size it's hard to top the new O-MD.