I think this is going to be a bit a niche camera. It improves on the E-M5 in lots of small ways, but for most users, and pretty much all new users, it doesn't really have much to offer over the E-M5. Videographers and hybrid shooters are better served by the Panasonic GH3.
The E-M5's IQ appears to be almost identical for most purposes. It has weather proofing and similar performance. It's smaller and significantly cheaper.
For those who own regular 4/3 lenses, the E-M1 will undoubtedly offer significant improvements in usability over current m4/3 bodies and it has a better EVF, but the E-M5's view finder again is perfectly good enough for most purposes. Those who own the 4/3 E3 (or other 4/3 bodies of that generation) will have a nice upgrade path, but E5 users primarily only really have an improved sensor. In initial reports, focusing speed is still not up to par with the traditional Olly DSLR bodies. It is evident from the lack of activity on this Olympus SLR forum that these users are now a small number.
The E-M1 looks to be a fantastic camera, but I can't see it being especially attractive to either "legacy owners" (those with 4/3 glass) or new fotogs moving to the m4/3 system. For both groups, the E5 and E-M5 still offer better performance or cost-effective options.
With the introduction of fast-ish f/2.8 zooms for m4/3 from both Panasonic (12-35mm & 35-100mm) and Olympus (12-40mm & 40-150mm), there's no real reason to purchase the original (albeit excellent) HG or SHG fast zooms because they are more expensive and significantly larger being designed for the flange distance of a camera with a mirror. People buying this 4/3 glass these days must be few and far between.
For me as a Panasonic G3 user, the E-M5 will become increasingly attractive as its price decreases with its attractive weather sealing and IBIS, but there's no reason for me to want the E-M1 as I sold my 4/3 kit a few years back. Besides, m4/3 offers a better selection of primes.
Historically, Olympus has been slow to introduce new sensors to its system, so until the E-M(2?) is released with a significantly improved sensor over the E-M1, I can't see it really catching on.