Hamachi is not open source. Not only that but it has two different levels, a free version and a premium version. Here's their comparison of free vs. premium:
http://premium.hamachi.cc/compare.php
Hamachi is a great idea and I have played around with it some; however, the free version lacks some basic functions which I have come to expect from small network file sharing programs. Example: The free version does not allow you to start or join a "group chat." The only way to communicate is through PM. Hamachi also limits speeds for free accounts and only allows a maximum of 16 people (17 including the "owner") per network. Up to 256 people can be in a premium network. There are many other limitations but I will let you read them for yourselves.
The thing that tops it off for me. Premium accounts are a subscription, it is not a 1-time payment deal. Premium accounts run at ~$40 a year. I'm not willing to pay that when I can use other programs that are open source and only require a little bit more effort to set up.
My overall rating of the program would have to be a 4/10. It's a good idea, but the free/premium limitations and the cost make it almost useless in my eyes.
-AngryGnome