Wow, its amazing no one knows how this really works..
Here is the break down:
You have a cell phone. it has an antenna. Said antenna is very tiny, and has no gain (increase in signal strength beyond standing wave)
To understand what I'm about to write, I must explain signal measurement.
Signal is measured in dBm. The iPhone 4 Shows 5 Bars at around -75dBm, and 1 bar at -105dBm.
Signal is measured in NEGATIVE dBm.
What this device has, is an external antenna. With a small magnetic mount antenna you can get between 2 and 5dBm of GAIN, which means it would add to the signal strength by -5dBm. So if you are at -105, it'll raise the signal to -100.
Doesn't sound like much, but every 3dBm of signal is DOUBLE what it was.
So the antenna adds about 5dBm, then there is an amplifier that adds around another 20-40.
However when it does this, it also amplifies the noise. So just like amplifying a very quiet old school audio cassette, you can HEAR it better, but there is a hiss.
This additional signal strength will up the bars to 5 obviously, but the quality of the signal is lower.
It is also worth noting that this amplification IS TWO WAY. The Amplifier will gain up the signal you send back, as will the antenna.
This is where the majority of the help is, since the tower usually looses your signal before you loose its.
These devices are known as Bi Directional Amplifiers. This one uses a small antenna inside the dock to get the signal to the phone after the amplification, and as such there is a lot of loss between the device and the antenna.
All in all, these will work great for keeping you in contact, but don't expect decent data rates in areas where signal is already sketchy to begin with.
Key point to remember: The signal is actually only amplified with decent quality at the antenna.. The powered amplifier will bring in noise with the signal..
So get a decent antenna to go with this and you may get decent results..