Sixt is great. They were flexible at the return location and time, and the vehicle they provided had just 2500km on it. Daniel Immonen at the rental counter was very friendly and helpful in answering my questions and making adjustments to the reservation. I've had good luck with them, and this was no exception.
The rental car return at Helsinki airport is hard to find, at least for me. The signage was not obvious, and the gps in the rental directed us back to the rental counter, instead of rental car return. We had to go into the airport and ask directions, and even then it was difficult to find. But this is more an airport problem than a sixt problem.
Needed to upgrade the car to fit two more passengers. That turned out to be impossible. All cars were so expensive that I had to cancel the reservation, with about 80€ penalty, and rent from a competitor.