Bit late but yes we used them last September for my brothers 30th flew into LAX (California) had a pre booked GMC Arcadia which he specifically phoned up to book months in advance with Sixt.
They shuttled us to the branch and the lot of them in the place were as bright as a black out..
Long story short they fobbed us off telling us they had upgraded us to a Kia Sorrento as they had no Arcadia and not to worry as Kia is a premium European Brand hahahaha quick check on the site showed the Kia was significantly cheaper per day they then put us in some crappy Cadillac and charged my brother nigh on £300 more than we originally pre paid.
Absolute Garbage don't use them.