It really depends on your situation. The problem with MUC is that it's quite a way out from Munich itself, so options for getting in are either to take the S8 train (40 mins +/-, though there are frequent reports of delays) or a taxi (€70). While the city is undoubtedly more interesting than hanging around in a deserted airport, €140 in cab fare, €200 for a decent hotel, etc add up. The Kempinski is the best hotel at the airport, though it also is not cheap. There is very little to do either in that hotel or at the airport. One idea may be to take a taxi to Erding, or even Freising, for example (each ca 15 mins in taxi), walk around, find a nice restaurant, and then come back to the Kempi to sleep.