Synchronizing my Mac OS X address book with my Windows Outlook contacts had been a major pain. I basically did it once over 6 months ago and then proceeded to only add contacts to my Mac - leaving Outlook out of date. As a result, my cell phone also had an old contact list since Windows Mobile phones do not synchronize with OS X.
With Plaxo Mac I just synchronized my address book on the Mac with my Outlook address book on Windows! Very cool!
It is a shame that it took Plaxo over 5 years to support the Mac, but I am glad they finally did it!
Now, if only I could get Skype 2.0 on my Mac then we would talking! (Power PC version, please)