No, I haven't used it. When I first heard of it, I called Lufthansa, and they didn't know anything about it. Told me to talk to my travel agent! Now I see it is on the Lufthansa website for purchasing flights. After you select your flights, there is a box on the RH side of the page to add seat reservations, rental car, or rail tickets. Rail&Fly tickets are 33€, per person one way, billed in USD.
The nice thing about Rail&Fly is that the price is similar to a Savings Fare without being bound to a specific train, which you could miss if the flight was late.
I wonder if you could book a flight to Frankfurt and then take a ICE to Berlin. It looks like you could. I think a flight to Berlin might cost more than a flight to FRA plus a 33€ R&F ticket.