My gf and I used it personally with the bumper case installed at all times and had no issues. I have sold quite a few of these already and never had one complaint. My room has bad reception issues (probably 1 or 2 bars at all times) and it had no impact after i installed it.
The case that had issues is their Vapor 4 model in the earlier days designed for the iPhone 4. That one automatically kills 2 to 3 bars once it's installed on.