Features and Benefits

Optical:
Increase maximum aperture by 11/3 stop
Increase MTF
Makes lens 0.64x wider
Patented 6-element/4-group optical design by Caldwell Photographic in the USA
Supports 4k video mode with DX lenses (except on GH5).

Electronic:
Autofocus
Can be powered either by camera body or by third-party external 5V power source (not included)
Iris/aperture controlled by the camera body
Smooth iris support with the latest Canon (2009+), Tamron (SP series 2013+) and Sigma (2016+) lenses
Image stabilization (IS) lens support
Support for cameras with in-body image stabilization (IBIS), including Panasonic GX7, GX8, GX80/85 and Olympus cameras
Electronic manual focusing (e.g. EF 85/1.2L II and discontinued EF 50/1.0L)
EXIF (focal length, aperture, zoom range)
Distance and zoom display on camcorders (requires lenses supporting distance information)
Cinema EOS lens support (CN-E 18-80mm T4.4 L), including AF (requires external 5V power through micro USB port), auto iris and servo zoom

Other:
New positive-lock EF lens mount.
Detachable tripod foot compatible with Arca Swiss, Markins and Photo Clam ball heads.
Rubber gasket protects MTF mount connection from dust and moisture.
The opening is flocked with felt material to reduce internal reflection.

Limitations:
Supports only Panasonic GH5, GH4, GH3, G3, G5, G6, G7, G10, GF3, GF5, GF6, GX1, GX8, GX80/GX85, Olympus E-PL7, PEN-F, Blackmagic Cinema Camera with Micro Four Thirds mount**, Pocket Cinema Camera*** and Micro Cinema Camera***
EF-S lenses require modification to fit or may remain incompatible even after modification (but third party DX lenses can be used without modification)
On GH5, Speed Booster XL can be used with full frame lenses only. DX lenses are unable to cover full-sensor-width readout + in-body image stabilizer (IBIS)
EF-S and third party DX lenses does not cover the full Micro Four Thirds sensor in still photo and FHD video (but will cover 4k video mode on GH4, GX8 and GX80/85).
No video AF for Olympus
AF may not work if the maximum aperture of the lens plus any EF Extender or other teleconverter attached is smaller than f/8 before boost or smaller than f/5.1 after boost.
AFC (continuous AF) and AFF (flexible AF) modes are not supported, except on Olympus OM-D E-M1.
Panasonic video AF may hunt more than a native lens or have unsatisfactory performance.
Some Tamron lenses may have unsatisfactory AF accuracy.
No support for Focus Stacking and Focus Bracketing on Olympus.
On Olympus, other than E-M1, you may move the single AF point among any of the 81, but you may not multi-point AF. There is no restriction on E-M1 and Panasonic.
Very early Olympus models (e.g. E-PL1) have no autofocus support.
EF-mount lenses are not silent but may make audible click noises during autofocus operation and iris changes which may be picked up by the built-in microphone of the camera. An external microphone is recommended for video.
Use of Canon Cinema lenses such as CN-E 18-80mm T4.4 L KAS S for still photography is not supported.
Iris changes are in 1/8 stop steps which may be noticeable in video recordings. Programmed exposure mode and shutter priority exposure mode should not be used for video. This limitation does not apply to Cinema EOS lenses which permit fine resolution iris control, such as CN-E 18-80mm T4.4 L IS KAS S.
No support for in-camera correction such as peripheral shading, CA and distortion