AT&T's MicroCell extends their famously inept network into your home or office. It does a tragically good job — you can use your broadband connection to not make calls rather than not making calls through an AT&T tower.
It's actually worse, because at least on the actual network you know when your signal sucks. With the MicroCell my phone shows five bars but will often refuse to make or receive a call. The first clue is often leaving the coverage of the device and finding out what you've missed. In fact, I think it might even be illegal under the Communications Act of 1934 as it's effectively a jammer.
Even if I can make a voice call the wretched thing screws up geolocation. In order to comply with E911 the device is registered to a specific address AND requires a GPS lock to function at all. Despite this it tells my phone that it's in Berkeley. Switch the MicroCell off and I instantly get the correct location in San Francisco. Switch it on and I'm teleported to Telegraph Avenue. My Google Latitude history has whiplash every time I leave the house.
Reading the AT&T forums I'm lucky. Other users are located in the wrong state, and have choppy voice when they can make a call at all.