In the early seasons, the show is really a story-of-the-week, and really almost a magic-event or magic-user of the week playing on the fact that magic had been outlawed in Camelot by King Uthur (played by Head), and anyone caught using it would be put to death. As the show went on, the series really used the larger theme of the good-versus-evil battle that would occur at the end of the series, with weekly tangential stories mixed in here and there. It is mostly a family show but does get darker over the course of the five seasons, so it is probably not something that very young children should watch. There is really no nudity or swearing in the show, but a couple of instances of sexual innuendo and then fairly tame kissing.
The complete series set is just a packaging of the individual seasons on DVD, with all the same extras that were included in the original release. There is a bonus disc that I think was probably original to the set that has more features like commentary tracks on a couple of the season five episodes, and a road-trip with Colin Morgan and Bradley James as they traveled around England to areas tied to the Arthurian legend. Luckily the show came out on DVD before the studios started skimping on extras, so if you like bonus content, this has a lot of it.
Overall, the series is very good. I think it finds its legs during the second and especially the third season and stays strong until the end. The acting and writing consistently improve over the course of the series, and as an added bonus, fans of Game of Thrones will recognize more than one guest star. It is definitely worth checking out.