Great service, poor location. Avoid the underpass.
Outside of a very long line to get my rental, my experience with Sixt car rental service was positive. We got an upgrade since the car we reserved was not available. Not the greatest car, but a good rental car. Clean and nice. Our return was flawless (we returned it to the attendant on the sixth floor just before closing time).
The biggest drawback is its downtown Manhattan location off of Battery Park. Returning the car using Google maps instructions from Brooklyn is a total disaster.
Most direct instructions put me on the Brooklyn Bridge to the FDR drive south, to the Battery Park underpass. It is impossible (especially with traffic) to get to Morris St this way due to traffic and police. To make matters worse, the lane threw me onto the Battery tunnel which is a toll ride back to Brooklyn.
Avoid my costly mistake and do this: take the Brooklyn bridge (if coming from Brooklyn), to Park Row (which is a circular right turn off the bridge). Take Park Row all the way down and make a left onto Broadway take Broadway about a quarter mile down to battery place. Make a right and take that down and make a right (and stay to the right on the NY-9A West ramp) and that will put you in the proper lane to make a right onto Morris st.
Wish the people working there would warn folks about this. But that's why you're reading this review.