I was in in Munich two months ago and visited the Museum and the Welt and rented an M4. I hired it for 3 hours so had it for the day. Drove to Salzburg, had a very quick coffee and look around, on the way back to Munich I left the autobahn for a while and went for drive through some of the small towns and villages. Very enjoyable day, traffic in and around Munich was very heavy though and there were roadworks on a few sections of the autobahn, annoyingly the first one was on the first section that should have been unrestricted so it was limited to 80km/h, but its not all about the speed.

It did take a while to get out of the heavy traffic so i was happy that i had the car for the day. If you can amend your booking and hire it for the 3rd hour and are happy enough with the added petrol costs then go for it. Best thing about the whole experience was the guy in BMW on demand who was checking the car before i set off, he finished up by checking thread depth on the car and asked me to try and keep it under 240km/h