I had a monster FM modulator that I got for dirt cheap on Woot, and eventually paired it with a 30-pin bluetooth reciever that I used up until this summer.
They do work, but it was a massive PITA constantly hunting for the best frequency to broadcast on and it always sounded terrible.
But, they do work. That flexsmart looks nice and it looks like it's easy to change the radio frequency on. I just wouldn't spend too much on it because you're not going to find much of a difference between a $30 setup and a $100 setup, IMO.