If you are headed to Weiz or any of the surrounding areas, this is the place to rent a bigger better vehicle. I was staying in small town where rental options are very limited and needed a large SUV. Rather than paying excessive airport pickup convenience fees (several hundred €/$) at VIE, I hopped on the train to Weiz Zentrum to pickup same lux SUV (BMW X5) for half the price.
FYI... The SIXT agency is located inside the Opal dealership and the sign is not easily visible from the road. My SIXT agent, Winklebauer, was awesome! He made sure everything was tip top before letting me take off in my big "American dream car".
I will definitely be renting from here every time I'm in SW Austria.