One of the exciting things about the Fujifilm X-Pro1 and X-E1 is how the cameras do away with the AA filter (also known as an optical low-pass filter). The AA filter in most digital cameras prevents nasty things like moiré artifacts from appearing in your photographs, but at the cost of some fine detail, so it always looks like there's a film of blurriness overlaid on your images.
A camera without the AA filter, like the X-E1, produces images which look that much clearer, that much sharper, with more clarity and crispness in the details. To be fair, if you don't pixel peep, you probably won't see the difference on Facebook. But if you do, and it's a fair bet that anyone who gets an X-E1 will, you'll certainly appreciate the difference.
The X-E1 can get rid of the AA filter because of its unique Fujifilm image sensor. Called the X-Trans sensor, it was first introduced with the X-Pro1. It uses a more random arrangement of RGB photosites than traditional image sensors, designed to mimic the random arrangement of film grain (more information and visual representation of this can be found in our earlier review of the X-Pro 1). The benefit of this unconventional arrangement is that the X-Trans sensor is less prone to moiré artifacts - they can still appear under certain conditions, but in all our shooting with the X-Pro1 and X-E1 we haven't seen any.
Another benefit is, of course, that high clarity of detail - the X-E1 scores a remarkable 2600 LPH (vertical and horizontal) on our resolution chart. It's also a solid low-noise performer, shooting at high ISO sensitivities that buck lesser cameras. With the X-E1, it's certainly possible to shoot up to ISO 6400 with easy confidence, the camera manages to keep noise down while retaining a high level of detail not yet seen in other mirrorless system cameras. Even ISO 12,800 is not complete rubbish.
Whether you'll like the X-E1's JPEG colors depends on whether you like the 'Fujifilm' look (or at least, the digital Fujifilm look). Colors have a blue-green shift, and the sensor's red channel appears especially sensitive; reds can appear quite intense in some images. The camera also has a tendency to let the whites blow, but to its credit they can often be recovered, even with JPEGs.
The X-Pro1 had a problem with color smearing, where a color would run over and contaminate adjacent areas. Because of the way the X-Trans' RGB photosites are arrayed, there are areas with no adjacent alternative color. For example, in a conventional Bayer array sensor, a red photosite may be next to a green, then a red. On the X-Trans sensor, you might have a green photosite next to another green photosite before you get a red photosite. So the image processor has to try and guess what color's actually hitting the photosite with less information to work on.
Color smearing doesn't appear in all images, but there are samples of it online. We haven't seen it in our own images, but we have seen what we can only describe as odd edge glows, where a halo or double-edge appears in certain subjects. It's not exactly purple fringing, as the glows aren't purple, and where we shot back-lit subjects there was no evidence of purple fringing. Again, it doesn't happen all the time, and you might not even notice it if you don't zoom into your picture. But if you do, or if you print them large, you might be able to spot them, and it's a strike against what is otherwise very good image quality.
Photographers who shoot in raw will also want to note that the X-Trans sensor's RAW conversion in apps other than Fujifilm's Silkypix seem to have problems getting rid of the color smearing. If you're someone who works exclusively in RAW and Adobe, the X-E1 might present a workflow hiccup for you (ChromaSoft has a very detailed, very technical report about this).
If you've read our X-Pro1 review, we said that its Achilles' heel was its sluggish auto-focus. In a pre-release briefing, Fujifilm told us that they put in a lot of effort to improve the X-cameras' AF and it shows in the X-E1. It's certainly snappier than the X-Pro1 when it first launched, it's able to find and lock onto subjects with reasonable speed. That said, it's also not state of the art fast in a year when we have Olympus, Panasonic and Nikon providing ultra-fast auto-focus systems that can even recognize faces (and in the Olympus' case, even recognize eyes – sometimes).
It also seems to prefer center focus, and sometimes will confirm lock when it hasn't really gotten one. Continuous AF is only center focus, and like on most mirrorless system cameras except Nikon's, is best left alone. It sounds bad, but the X-E1's AF is usable for photographers who shoot at a considered pace.