We rented from Sixt in Israel last summer and I have to say while they're not the most high end agency they provided great service. Our van had an overheating problem...it was 100 F outside normally so it's pseudo understandable since we were driving in the mountains. We returned that van and were given a bigger-nicer van, a free tank of gas and 1 day credit for our problems. the switch took about 20 minutes.
We didn't have a fuel charge problem even though we did return it a little less full than required.