I bought the Viltrox EF-EOS M2 and the EF 17-40mm f4
The adapter weights 140g and the lens is about 480g, so together it's just over 600g. It does look a bit odd but it feels alright due to the M6 having a decent grip for a small camera.
Build quality of the adapter is good. There's no play on either end and it's not too tight either. It feels solid yet is quite light (most speed boosters are over 200g).
The autofocus works really well. Pretty similar to the native 22mm, just a little noisier. It can be heard in video if the room is silent but otherwise it's not a problem.
Central sharpness is good. Edge is poor up until f8.
Weirdly, the edge sharpness at 40mm is worse than at 17mm.
Overall the adapter makes a great cheapskate option for full frame but I've realised it's a bit surplus to my requirements right now so im returning it. And with lenses like these it's better to have a camera with a bigger grip.
17mm @ f2.8:
40mm @ f2.8: