I visited the Hedeby site, which is well developed for tourists, with a museum (closed at the moment for refurbishment), walking pathes and reconstructed houses. Hedeby (or Haithabu in German) was a well-sized city at the edge of the viking lands. As with other viking sites (like Birka or Trelleborg) the problem is that almost everything was built out of wood, so there is not much left of it but the earthworks. In the case of Hedeby, the earthen city walls are still standing (and walkable), but most of the area inside is empty. There are some quite nice reconstructions of viking houses, which will be the most interesting thing for most visitors. There are also demonstrations, market stalls and exhibition houses, so there is more to do than to stare at mounds. Also interesting is the visitable archaeological dig, although most finds seemed to be small things like nails or bone remains. The whole area is rather idyllic, with a nice lake and an undisturbed environment. It is a good place for hiking. As there are no nearby settlements today, getting there can be a problem. It is rather easy with a car, as the main highway from the north (e.g. to Jelling) to the south (e.g. to Hamburg) is quite close, but public transport might be difficult.
All in all, I had a very pleasant visit there, and really got a good impression of how they lived in the viking times. However, the most interesting things are reconstructions (Kind of like Unteruhldingen with the pile dwellings). Therefore it is hard to judge the worthiness and OUV of the site.