I am committed to Sixt for the following reasons:

1. As an LH SEN I get an automatic Platinum Sixt card
2. As an LH SEN I get a SEN Gold Visa Card which covers my comprehensive insurance when hiring through Sixt.
3. I live in Germany (Sixt's home market)
4. When hiring from Sixt I get the 35% Platinum discount + free insurance (exclusiv) rate.
5. I ALWAYS get bumped, as a regular hirer (I'd qualify for gold or plantinum most years as I don't own my own car) I know the guys at FRA zentrum and they just give stuff away to me (or so it feels).

I've had bad experiences 2/2 with Sixt AMS and mediocre with Sixt STN 2/2 but LHR has always been 2/5 fine or 3/5 really good.

If I had to complain about one thing, its their lack of 'on airport' parking or out-of-hours opening times last time I had to drop a car on a one way at Thrifty (do a runner) and call Sixt LGW from AMS and tell them the car was in their competitors car park because I wasn't going to drop it in Redxxxx? and get a taxi to the airport for 25 quid just because their helpdesk claimed they were 'at' the airport. The LGW was used to this though