I got my flat metal back for my iPhone 4 in today.
Signal loss is minimal, and comes and goes (sometimes i have a small loss, sometimes i don't). Not a big issue.
The biggest issue is using the Flash on the camera. For close up photos, there is no issue, but for scenes like dark rooms, the Flash reflects inside of the camera and appears to create a blue haze on images.
I took the metal back off and looked at the area around the camera lens on both the glass and metal backs (I am looking at the INSIDE part of the back). The original glass back has a raised up area for the camera lens which appears to separate the LED flash from the camera. The metal back does not have this, and as such the LED flash spills over from within the phone itself.