Hi mate. I use micro four thirds for my base for digital. I have 3 mFT cameras, GH2, GF3 and Blackmagic Cinema Camera.
I then have several mounts, some expensive some not so much. Cheap ones are MFT to M39, M42, FD. More expensive is MFT to Nikon G as it has an aperture control lever. The most expensive are the Metabones Speedboosters as they contain an inline condensing lens designed to use more of the image circle that is projected from a lens designed for a full frame camera onto a smaller sensor.
Crop factor on mFT is typically 2x. With the Speedboosters (0.71x) that becomes 1.4x. Net result is more light, so faster lense, wider field of view and resulting narrowing of depth if field.
Speedboosters are around £350, but if you think about what it does to all my lenses it's pretty good value. My 1.4 becomes a 1.0, 2.0 a 1.2, 2.8 a 2.0. That makes seriously fast glass for little outlay.especially with my Canon FDs.