Body & Design

In common with Olympus' range-topping Four Thirds DSLRs, the E-M5 is weather sealed, as is the accompanying flash unit. The 12-50mm F3.5-6.3 kit zoom is similarly sealed. Additionally, owners of high-end, weather-resistant Four Thirds lenses will have the choice of buying the MMF-3 adapter - a sealed version of the Four Thirds to Micro Four Thirds adapter.

The E-M5, its add-on flash, optional grip and primary kit zoom lens are all rigourously gasketted to help keep water and dust out of the entire package.
There's even a version of the Four Thirds to Micro Four Thirds adapter with seals to prevent unwanted ingress.

Compared to its rivals

The E-M5 may have borrowed the style of a DSLR but it's a lot smaller than one. Here we compared it to the Canon EOS 1100D (Rebel T3), itself not a particularly large example of the breed. As you can see the E-M5 is smaller in every respect, despite offering a much greater degree of direct control. The Rebel has a built-in flash, which the Olympus lacks, but the clip-on unit is so easily fitted in a pocket that it makes almost no difference when carrying the cameras.

Olympus
PEN E-P3		 Canon Rebel T3
(EOS 1100D)		 Panasonic DMC-G3 Sony Alpha
NEX-7		 Olympus OM4

The Olympus is a touch larger than most mirrorless cameras (including the NEX-7 which also has a built-in viewfinder), but it's small compared to an DSLR.

Compared to the Panasonic DMC-G3

In simplistic terms, the Olympus E-M5 and Panasonic DMC-G3 are pretty similar cameras, offering similar 16MP sensors and built-in viewfinders. But, for the extra money, the E-M5 offers twin control dials, a magnesium alloy body, weather sealing, in-body image stabilization and a 1.44M dot viewfinder (rather than a 1.44M dot equivalent, field-sequential design).

And, of course, once you've considered those specification differences there's also the aesthetic difference between the two, with the OM-D having a more distinctive look.
The E-M5 features more control points than the G3, with Panasonic relying more on the touchscreen including a configurable on-screen function menu to help close the gap. The E-M5's screen, meanwhile, is higher resolution and based on an OLED panel, for a brighter, wider gamut image. It only flips up and down, rather than swinging out from the body.

The E-M5 has an eye-sensor to switch automatically between EVF and rear-screen use - something the G3 sorely lacks.