They're small, sexy and need specialised lenses. But if you want superb image quality and DSLR control in the size of a compact, there's no other option.
Micro 4/3rds cameras are confusingly known by different names according to the manufacturer. Sony describes its 4/3rds as having an E Mount system, Panasonic calls it a G Mount System, while Olympus refers to the technology as PEN and Samsung describes it as NX. Ricoh has gone a slightly different way with a combination of camera + body that slide into a camera back.
In simple terms these hybrid 4/3rds systems are a marriage between a standard compact camera body and a flexible DSLR type interchangeable lens system - without the overhead of mirror and multiprism. As such, you get the best of both worlds: a lightweight and compact camera which has the luxury of multiple lenses for differing situations – all without sky-high top-end DSLR prices.

Olympus' PEN-EP1: DSLR control in a body not much larger than a compact camera.
The range of lenses for 4/3rd cameras is growing all the time, allowing the dedicated photographer to build a versatile system to suit his or her needs. For example, a sports photographer may need long distance and portrait lenses and so can ignore any macro lenses.
But the big, big factor is the size of the sensor. While allowing more light to be processed, a larger sensor also gives the operator a much bigger choice of depth of field – that ability to “pop” a subject into focus in the foreground and away from a blurred background. The result also gives much richer colours and skin tones. Unlike 4/3rds, standard compact cameras with smaller lenses and even smaller sensors simply cannot do this.
It is an effect that is used in all photography art forms, ranging from sport (say, the frozen race car or horse with a blurred background), portrait photos, still life, close ups of flowers and insects and even industrial photography. And with the Live View function (which shows a true image in the LCD of what the sensor sees instead of one composed by the camera) the image can be perfectly composed via aperture, shutter speed, white balance, focus, special effect and so on before taking the shot.
Be aware, though, that one thing you don’t get with 4/3rds is a viewfinder, so if you don’t like using an LCD/LiveView combo, this may deter you. Like DSLRs and unlike compact cameras, 4/3rds cameras have dust reduction systems too, preventing the smallest of particles from intruding onto the picture. Ricoh’s unique slide-in body is also fully sealed.
Different manufacturers have addressed the control systems of the cameras in their own way. If you prefer a more tactile feel, Sony’s use a combination of physical wheels, soft keys and switches which might be more to your liking than the Panasonic’s more touch screen orientation. All models have an extensive feedback system via the LCD – or in the case of the Samsung, a faster AMOLED screen – giving all relevant details of the current camera settings at a glance.
Pricing is pretty consistent across all brands. Starter systems with a body and single lens kit cost around $800, while more advanced models are around $1,000 (e.g. Sony NEX3 versus NEX5). Extra lens kits sell for around $1,200.
If you are a practised DSLR user, then the 4/3rds style of system may seem like a step backwards. You already have a large investment in lenses and cannot see what advantages you may get by adding one of these format cameras to your arsenal.
On the other hand, if you are a compact camera user, want the flexibility of the smaller body size that format offers, don’t mind the lack of viewfinder and are happy to be “restricted” to Live View and also want to be able to select a different lens depending on conditions, then the 4/3rds type of system may well be right for you.
Image quality is superb and comparable to that produced by DSLRs. The larger sensor gives results that compacts can only dream of and more and more lenses will no doubt be released as the interest and take-up continues.