I was apprehensive about the car hire before arriving because of the reviews i read across all companies, but we were delighted with the car we received, in what was a swift pick up/drop off. We booked a mid range SUV and got upgraded to a Rav4 hybrid, which was very smart, spacious and comfortable. The winter tyres effortlessly dealt with the heavy snow and ice, which is comforting when transporting a young child.
Just be prepared to receive some small additional toll charges/auto pass fees after hand over, which are unavoidable, although shouldn't account to too much in total. It's an automatic payment when you receive your final invoice. I don't think these hidden charges are exclusive to sixt either. Just make sure you read all your paper work, terms and conditions etc before arriving so you know what you're covered for, and take plenty of photos inside/out when you receive the car, which they did advise on when I picked up the car.
The security deposit was returned within 48 hours, despite being told 3-5 working days, which was very pleasing.
We would recommend Sixt in Tromso, and plan on using them again for future visits.