Here's a quick comparison of the unedited images from the Olympus OM-D EM-5 and the Fujifilm X100S cameras.
It is very easy to love the Olympus OM-D - you almost certainly get usable images all the time with its superfast AF and IBIS image stabilization, which in my opinion, is unmatched in any of the cameras I have used or tested.
Plus, another point in its favor is its relative light weight, compared to the larger APS-C and full-frame (FF) DSLR cousins. Sure, you can always get better image quality (IQ) from larger cameras, but to lug around heavy gear is not fun, and for us who started out as street photographers, large DSLRs intimidate people, and are hard to whip out of your pocket (you couldn't - they are too huge) when you need to.
The OM-D is also one of the best HD videography tools around, especially because of its IBIS in-body image stabilization, almost eliminating the need for serious stabilization gear. I have the Panasonic Lumix GH2 and the GH3 as well, and still the OM-D trumps the Lumix cameras because of in-body image stabilization. And still images from the OM-D beat the Lumix GH2 and GH3 all the time.
The Panasonic Lumix GH3 is a great HD videography tool simply because it has dedicated videography features, has a much-needed audio monitoring via headphone, and does not require a custom adapter like the OM-D for audio input - you simply plug in a microphone on the hotshoe and plug the 3.5mm plug into the GH3's 3.5mm audio input port and you are set. For image stabilization on the GH3 (and GH2), you need Lumix lenses with the Mega or Power OIS, because the Lumix cameras do not feature in-camera, but rather, in-lens stabilization.
So, the gist of it is, the OM-D, despite being "just" a micro four-third camera, is a competent and touch challenger to many cameras, including APS-C sensor cameras.
Having owned the earlier Fujifilm X100 rangefinder fixed 23mm lens (equivalent to the 35mm lens on a full-frame system), I got hold of the newest X100S, which has received quite good reviews worldwide. It is a lightweight rangefinder with great looks, and has an indescribable image quality, not quite Leica-like, but very likable.
The Fujifilm X100S (and X100) has a F/2 23mm lens, while I used the Olympus 17mm F/1.8 lens on the OM-D for this brief test. Both lenses would produce images corresponding to the 35mm perspective on a full-frame system.
At the widest open aperture of F/2, the Fujifilm X100S seems to produce very soft images with a "glow", which some people would love, and many may hate. The images sharpen up a great deal at F/2.8 instead. For the Olympus 17mm with F/1.8, you can get very sharp images at the focal point, but for the test, I set the lens to F/2 as well. I also shot a series of images on the Fujifilm X100S at F/2, F/2.8, F/4 and F/5.6 to show you how the images sharpen up. I am a little disappointed that the camera does not seem to give me sharply focused shots at the widest F/2 aperture, because all the prime micro four-third lenses on my OM-D and Lumix GH3 can do that at even wider apertures. In fact, the images on the Fujifilm X100S at F/2 has a similar look to images at F/0.95 on my Voightlander 17mm F/0.95 lens (of course, the Voightlander 17mm lens is heavy).
In this outdoor shot, you can see that the OM-D produced images with great color dynamic range and contrast, and at F/2, the images are sharp enough at the focal point, while blurring out pleasantly outside the focal point. The image from the X100S has a "softness" to the image, with less color range and contrast. Both images are pleasant, but perhaps one could say the OM-D image has more contrast while the image from the X100S is softer and "gentler".
In this indoor shot of the Muay Thai boxing glove, the OM-D produced a sharper image that holds up well under magnification, while the X100S produced a softer image with a faster "fall off" away from the focal point, and an image with a warmer, brighter feel, compared to the OM-D. One would love one and hate the other, although both images are not bad.
In this shot we look at how the Fujifilm X100S produces images at different F/stops, where the F/2 seems to have very soft images while at F/2.8 onwards, the image sharpness becomes much more preferred by many.
Again, I would have wished the X100S can produce sharp images at its focal point even at F/2, but one can't expect everything out of a single camera - no camera can have perfect technical or image features.
One of the most important considerations is that Fujifilm switched from a Bayer to a proprietary X-Trans sensor, which makes their RAW (.RAF) files incompatible with many RAW converters and processors out there. On the contrary, RAW files from the Olympus OM-D (.ORF) are easily opened, viewed, converted and processed from simple converters like RAWKER and more advanced ones like Iridient Developer. (Update on April 18, 2013 - Apple released its Digital Camera RAW Compatibility Update 4.05, which has compatibility with Fujifilm X100S, X-E 1, X-Pro 1 and X20. Problem solved).
What are my initial thoughts?
If I have to shoot fleeting moments that demand speed and repeatable results, and I can only carry one camera, I would bring the Olympus OM-D EM-5 with a nice wide angle prime lens. The camera is very easy to use, and very easy to produce pleasant and usable images. The Fujifilm X100S is a lovely camera that I know I can love, but perhaps it is better for those times where I am contemplative, at ease, and in no hurry to take images. For example, in a quiet afternoon, I could bring the X100S out, and walk along old roads, quiet alleys, old houses and estates, and green nature. In an unhurried environment, the Fujifilm X100S would be a beautiful camera to use.