Like almost all modern lenses, it's gonna be good. At $559 and 7.6oz, not exactly cheap & ultralight though.
The Sigma body won't come cheap either. Could possibly be $1500-2000. If not shooting video maybe better off with a used a7, which is a bargain at the moment.
And of course there's no way using this on the M. Mechanical adapter could be made, but no infinity focus since the L's flange distance is shorter.