I have used Fastlane (the name Sixt gives to the app check-in) about a dozen times. I would say I had issues about 50% of the time, but it has only been available to my area I would say for 12 months.

You can select a vehicle 30 minutes prior to your reservation pickup time. The app would provide sometimes free upgrades too. I have noticed that, if you have a status with Sixt, the upgrade is not systematically provided to you even through the car park inventory suggests that there is availability. The pool of cars available through Fastlane is different to the regular pool of cars. When you have trouble with checking-in, no-one can troubleshoot issues at the car park and I was often sent back to the car rental front desk to be checked-in (on a different car) the classic way.

The system got better over time. All in all, it saves me a lot of time compared to the traditional way Sixt does check-in. Sixt cares little about status when you check-in the classic way. There are dedicated lanes, however I have never found them to be respected. Effectively, for these of you who have ever rented cars in the US, using Fastlane is nothing different to be a Hertz Gold Rewards members for instance and going to your car straight away without having to talk to anyone.