new Metabones adapter gives nex crop factor higher-quality full frame look.

It is definitely a curiosity - on one level, I like the idea. I certainly welcome this over the plethora of teleconverters I've seen!

Alas, I'll probably pass. I've never been a huge fan of aftermarket adapters and/or converters that contain additional glass - not sure how this add-on might mess with the elegant simplicity of an old 35mm prime lens' optical design. If corner performance is already somewhat compromised with a plain, cropped rendering (as is very often the case IME) I don't think it will be very pretty after you optically divert material from the outer extremes of the lens' projection circle.

Especially for the EOS system, for $600, I think I'd rather just get an old 5D body.