It looks quite compact until you fit the hood

With regard to the focal length, being pedantic I guess I would phrase it as the same field of view as 600mm FL on a 35mm frame

I have used it with a single Canon FD x1.4, here's an example
Robin by
Huw Prosser, on Flickr
I have also used a x2 Tokina and the Canon x1.4 for Moon shots giving around 840mm on 35mm.
That most recent Moon shot was taken with my Oly MC-14 and the Canon x1.4 to give around 590mm on 35mm with the Tokina.
Using it with the E-M1 is a breeze, peaking or zoom to x5 or so if necessary.