Though question to answer. While in general it is some kind of national sport to complain about Deutsche Bahn and its on-time record and reliability it is actually reliable. As ! said a 5-15 minute delay is quite common and you should never book a trip where you have less than 10 minutes changing trains. Said this, there is always a very slim chance of longer delays or even cancellation due to technical problems, weather or blocked tracks (almost the same risks as if you fly) . The only way to be 100% sure to arrive on time would be to take a train the evening before and staying the night close to the airport. In my opinion it all depends what would be the consequences if you missed your flight. Is it just a minor inconvenience than I would not be worried about the reliability of Deutsche Bahn. If on the other hand missing your flight would cause some big problem for you (missing important business meeting, being late for an important family event) you should investigate alternatives and/or have a back-up plan (for example somthing like ! outlined).