Am I missing something? Rated 3 out of 5 stars
It looks nice, but...
I installed Siphon on two computers: Ubuntu (A) and Windows XP (B), both with Firefox 3.5.4. Registered & synched from A.
Logged in & synched from B => Addons from A show up. Set them to "ignore" as they are Linux specific.
Synched again from A => "No addons need to be installed". Addons from B don't show up. Restarting Firefox doesn't help.