Well, the best viruses are the ones that are able to spread to machines without some kind of user intervention. Typically taking advantage of a service exploit (such as the rpc buffer overrun).
In the case of Sobig and it's predecessors (like Klez, Mimail, etc.) it takes advantage of human curiosity. And it's very hard to trace back because of the random address book entries. The damage is low but the annoyance is high. Same with Blaster.
Just be glad that the authors didn't add a payload like Chernobyl. If both the Blaster virus and Sobig worm had been set to destroy the file system, huge chucks of the Internet, home users, corporate networks, etc. would have been wiped out. There would have been a surge in I.T. technical support for a few months with much overtime expenditures and downtime - it would have been a catch22 for the economy - both a boost and a disaster.
