desy,
The official Napster server obviously has more users than the OpenNap network (where a bunch of opennap servers are connected to each other a la IRC).
However, there are still thousands, and that's plenty to find what you want. I do not by any means consider the official napster more "reliable" than opennap.
Remember before Napster got stayed and it was announced that it would shutdown in 24 hours? The opennap servers at that time were flooded with many more people, who eventually left after a few days back to the official Napster after it stayed. I see no reason why this wouldn't happen again (only on a more permanent basis) if napster shuts out the free users.
I'm not condoning it either way, I'm just stating what will happen.