I'm a history buss, and I think it's interesting. The Viking time period is not a period that seems to get much attention, and I'm glad they decided to place Beth and Patrick there.
The books are simple, but seem realistic (other than the fact that time travel is impossible). Beth and Patrick wear costumes so they fit in semi-well, and they never seem to be able to change the past. They always find what they're looking for. The books also emphasize God more, although it seems to be everyone except the two children who refer to Him.
The characters are not always fleshed out, but given that it is for younger readers, that could be overlooked.
The content is interesting and gives information about a historical event often overlooked by writers and researchers.
I would say a 9 on a 10 point scale.