I'd probably disagree.
Depending on how it's used, it would work, but probably not the way you are thinking.
What those units are is a Bi-Directional amplifier (BDA). They'll take a signal in their passband and amplify it. For the incoming signal from the cell tower, it amplifies the signal and reradiates it inside the vehicle. For the outgoing signal from the cell phone, it picks up the signal from inside the car, amplifies it and reradiates it using the outside antenna.
An important part of the way these things work is that there has to be some separation between the inside unit and the outside antenna. If there isn't enough separation, the unit will go in "self oscillation" and shut down. Self oscillation is where the inside unit will pick up the signal radiated by the outside antenna, amplify it, send it to the outside antenna, and the whole process repeats. Sort of like getting a microphone too close to a PA speaker, you'd get a feedback loop.
Modern BDA's are required to have an automatic shutdown on them to prevent this from happening since a self oscillating BDA can wipe out an entire cell site.
Anyway, since the filtering on these things isn't very good, they'll often pass the public safety portions of the 800MHz band just fine. It can work by boosting the signal from outside the car and reradiating it on the inside. We used to have some high end BDA's installed by Nextel at work in some of our basements. They were designed for the NexTel service, but they worked well for my 800MHz trunked system. But, there's a big difference between one of these consumer units and a commercial grade indoor BDA system.
For these things to work, that has to be some sort of signal available for them to work with. They won't pull a signal that just isn't there.
So, if you have a usable signal outside the car (or home), then it's much easier/cheaper/wiser to just work with that and skip the BDA.
As cmjonesinc said, the best solution is to mount an outdoor antenna. If it's on your car, then do it right, a permanent mount 800MHz antenna, installed correctly, and connected directly to your scanner.
If it's at home, then put up a proper antenna and use high quality coaxial cable and connect to your scanner.
For the $179 cost of that unit, you can get a good antenna, good coax and do it right. You won't be limited to the 800/1900MHz band, using the right antenna you can boost performance for all the bands you listen to.
Also, I owned a Wilson mobile BDA unit many years ago. It worked very well for what it was designed for. However the quality of the unit really sucked. Cheap plastic, flimsy construction, etc. While it did what it was supposed to do, the external antenna on it's own was what made it work so well.
