I'd like to help you out with the voice of experience since no one else has responded, however, given the camera already has a vf, I've never felt compelled to get an external one. I invested in an adapter and polarizing filter instead.
Actually, I was talking about an optical viewfinder. There is a post on the internet about a pro using one on a G10, a 35mm OVF, and matching the zoom to the view on the OVF.
As you know, the viewfinder on the G12 only shows about 80% of what the sensor captures, so I thought maybe an external viewfinder might be alright, but they're not cheap, and I thought I'd see if anyone had actually tried it.