As noted by the other reviewers, the aperture of the EF lens in use cannot be changed when the lens is mounted on the micro 4/3 camera body, you also lose autofocus capability and the image stabilization function of the EF lenses. Those are the downsides to this adapter. The upside is the ability to use a wide range of Canon EF lenses on you Olympus or Panasonic micro 4/3 bodies. The best benefit here is the use of the long lenses and EF tele-zooms. You get an effective doubling of the focal length with no loss of lens speed. My 70-200/2.8 EF lens is now the equivalent of a 140-400mm zoom with the same maximum aperture of f/2.8 when used on my Lumix G1. In addition, the minimum focus distance of the lens remains the same. Not a bad compromise, in my opinion. Of course, you have to be aware that this doubling of focal length effect also doubles the likelihood of camera shake when handholding. Consider a tripod or use a higher ISO.
I've only used this adapter a short time so I can't judge long-term function, however, it seems to work well with the EF lenses I've tried on it: 50/1.4, 85/1.8, 70/200/2.8, 400/5.6 and 100-300/4.5-5.6. I've not tried the trick of stopping down the lenses on an EOS body and using it at an aperture other than maximum. That seems a little too involved to me although I can see how it would be a benefit under some circumstances--high speed lens in full sunlight, for example. Since I've not used it a lot and don't know how well it will hold up long-term, I'll only give it 4 stars.
If for no other reason than to experiment and have some fun, I would recommend this adapter.