We rented two cars some weeks ago. I have to say that we had an incredible experience renting in Sixt (Guimarães), nothing went wrong and we had no issues/problems.
The service was amazingly conducted by César Ribeiro. I would say that it was the best service of Renting a car I had during my life (in different countries). He was extremely attentive, polite and efficient.
I would recommend it! The prices were very competitive and the service all good.