I bought this iphone cradle 3 days after I had the car and I noticed an improvment to the signal as I have o2 here in the UK and it's not so hot but I notice 5 bars in the cradle and 2/3 bars just on top of the arm rest.
The cradle only charges the phone and uses the outside antenna everything else is via bluetooth - I use a seperate ipod with the little 'Y' cable for my music - hope i've answered your question