to be successful, need good (if not great AF) and an adaptor (already have this, but needs to continue to work) for EOS EF and EF-S lenses.
Why would I want to use BIG/HEAVY DSLR lenses on my mirrorless cameras?? When I dump my DSLRs the lenses will go as well. For me, NO adapter wanted or needed. YMMV
Depending upon the crop, my 500/600 could reach to nearly 2000mm equivalent
Less equipment to carry - used the M as a backup body. Have M specific lens for sure, give it more functionality,not less