So, how well does it work, compared to the Canon EOS we tried last?
Here we have a Panasonic DMC GF2 M4/3 camera, you can see the exposed sensor (thank goodness it has a dust-reduction system) and lying next it an example of a modern M4/3 lens, in this case the wonderfully compact 14-42mm "powerzoom" lens. Below it lie an old, worthless, M42 lens - an unremarkable Wiestar 35mm 2.8 in this case - and a cheap M42-M4/3 adaptor. Unlike the M42-EOS adaptor, this one is unfortunately quite deep. In the leading picture we can see this lens fitted.
So, how does it work? Handling is a little awkward due to the small light camera with the long heavy lens jutting out ahead of it - 2cms of essentially hollow adapter does not help - the center of gravity is rather far forward. Aside from this - rather well. The LCD screen on the GF2 is good and the ability to manually focus using this screen with a zoom focus preview function far exceeded my expectations. To my amazement I did far better with this camera-lens combo on a day out than I did with the EOS 300D and manual lens. Nonetheless, forget about taking pics of toddlers with this, not ones that are awake in any case. As with the EOS you get through the lens metering.
The smaller sensor of most MILCs (Samsung, Sony and Fuji use a APS-C sized sensor, but the M4/3 is smaller, and the Nikon 1s and some Pentax MILCs are smaller still) and other factors means that even more obscure old lenses can be used, for instance C-mount CCTV lenses or M39s. This is fun but the smaller sensor brings with it a rather large problem. the crop factor. That nice wide 35mm...its equivalent to 70mm on this camera. Seeing as most old lenses you are going to be using were intended for 35mm film cameras, they tend to be in "longer" focal lengths. 35mm, 55mm, 85mm, and 135mm seem to be the most common sizes for M42s for example. These all end up being rather long on the GF2. Pity.
In the end the most useful lenses for the M4/3 cameras are the wonderful modern pancake lenses.
Finally, largly for fun, a M39 Jupiter-8 50mm 2.0 lens from my Zorki, working well on the GF2 with a rather compact L39-M4/3 adaptor. Quite nice and compact - and you will simply never get a M/L39 lens to work on an DSLR. But 100mm effective focal length...well look at it this way: For a 100m f2.0 lens, its very small and light indeed!
To summarize
Using M42 and M39 lenses on Panasonic/Olympus M4/3:
Aperture preview: No
Camera aware of aperture: No
Metering: Yes
Available modes: M, A