We had the opposite experience, with a 2-year-old - not that they were unpleasant or anything - but it was still not a good experience. This was LHR-ARN.

Our child had a purchased seat (as we had always done, even before age 2). We had also always traveled with a carseat.

SAS would not let us use the carseat. They insisted we check it. Then they lost it, leaving us with no carseat at our destination.

Bad experience.