Netgear is a very popular brand, and it's inevitable that a proportion of its users will have something bad to say. What I can say about the models mentioned above is that we come across a very large number of satisfied users here. Actually it's probably not unfair to say that most Broadcom-based routers have much the same ADSL performance, so the differences are in user features and to some extent the wireless capability.
Concerning user features, the Netgears have probably the most user-friendly configuration interface, and also have the possibility to use the custom DGTeam firmware which offers many more features.
As to wireless, two of the models mentioned, Billion 7800N and DGN2000, are wireless-N capable which possibly gives them a rather better range.
For what it's worth, I'm currently using a Billion 7800N, and for me it's as good as any router I've ever used for speed, stability and all-round competence, but that doesn't necessarily mean that everyone will have the same good experience.