I have never been to the Kempinski Hotel but have been reading up on it. From my readings, this is my assesment. Please let me know if it is accurate or if I have missed something.

If you fly FC through FRA you can use a day room at the Kempinski Hotel for no charge. It looks very nice. However, with the new FC terminal/lounges is it really worth the trip? I guess if you need a real bed for a few hours, a swim in the pool or the health club then the answer is yes. However, there is no free food, drink or internet access which the FC terminal has. If you want to go to town, then the Kempinski is out of your way. You can't even use it as a base while seeing a few things in town.

However, if you just want to relax, get something to eat/drink then use the FC terminal. The terminal also has a shower and a day bed to catch a little sleep. If you have a long layover (I'm seeing a lot of people with 6-12 hr layovers), you can use the terminal to get something to eat/drink, go to town, and still be back in time to use the terminal again before being driven to your plane.

Does this seem accurate?