The addon has tons of potential to be very useful. There are a few problems I experience with it, however. If you 'Delete' an addon, it will delete the addon from the current installation. Unfortunately, this doesn't seem to remove it completely as a regular 'Remove' would do from the addon manager. Can't say what it's missing, but it is definitely leaving something behind. I usually have to re-install it then remove it through the addon manager. It's not rare that there's an addon that will refuse to be downloaded from the Siphon installer.
Naturally, it does not synchronize addon settings, though this is to be expected. It'd be very useful to have that added as an option, but it wouldn't be fair to expect them to add it considering how much space that would probably take up on their servers. Though I'm sure adding Stylish, Greasemonkey and the like synchronization might be a little more reasonable.
The addon at it's current state is useful as a reminder as to what addons you have yet to add and most of the time as a quick shortcut to downloading them straight from the manager. I'll continue using it with hopes it will be improved.