They work in the sense that your repeater has a good signal as well. The problem with repeaters is that the antenna needs to have a good signal as well. If not you will end up with your blackberry has a good signal but your repeater has a bad signal so you are back to square one. So make sure the repeaters antenna is high enough to get a good signal. If the general coverage area is bad to begin with, then you are taking a risk. If you are buying this because you live in a house with aluminum siding and aluminum window screens or you basically live in a Faraday cage, then a repeater is for you.
Faraday cage - Wikipedia, the free encyclopedia
Wilson makes some good repeaters and antennas.
Cell Phone Antennas and Cell Phone Boosters at Wilson Electronics