I use them as well in additon to the base coverage I have through renters replacement cost insurance (doesn't provide much for guns) and NRA free insurance (need to send in forms) for being a member. They also advertise in Shotgun News (how I originally found them).
I've also found that some insurance companies aren't all that interesting in letting you get a rider for increase coverage. Mine came back with an outrageous number whereas my brothers (different insurance company) was quite reasonable.Most homeowners/renters policies cover guns, but they typically have a separate and much lower limit for guns (like $2000-5000). Double check the fine print for the gun limits.