If you're AT&T, get a microcell.
Dont.
at&t's MicroCell uses your personal broadband (cable modem, etc) to carry their signals, instead of a tower.
The microcell essentially is a little "cell site", but not connected to their backbone like a regular tower.
That sounds like a cool idea, using your own bandwidth to take some of the load off of at&t, but when you get to the fine print you will find you are charged your normal airtime minutes even though you arent using their tower.
And on top of that, you also pay for the privilege of using the MicroCell itself.
Essentially, you are paying at&t to use the bandwidth you are already paying for elsewhere.
Verizon has a similar setup, called FemtoCell, and is as much a ripoff.
I read a analogy that seemed to explain it well:
You buy a city bus pass, but the bus is full, so the city rents you a bicycle you use to follow the bus to your destination. You pay twice for less service.
Go with the cell amplifier.