I dont have much experience using both Plaxo and LinkedIn. I did find both networks easy to use, and I can certainly see how both would be useful in my professional field.
Plaxo is a great way to store contacts and addresses automatically. I often find that with email addresses I dont save them or find it difficult to access the contact's details. With Plaxo, I have found it interesting that when an update is made to a person's contacts, it immediately updates that information to the entire contact group. Pretty cool stuff!
LinkedIn seems to be a very useful and practical too to use in the professional environment. I found it interesting how easy it can be to include contact info, previous work experience, certifications, etc. I also found it interesting to research and look up various companies and find out more information about them. This could be very useful when applying for jobs, or trying to do some background research to learn more about a company in general. Very neat!