I think, for now, a standalone plugin is best. I think, over time, it'll make sense to start consolidating some of these things into a common library of plugins. Many of my plugins are extremely similar in their implementation.
I actually think that it'd be cool for the Watching, Read, and Food plugins to detect if the Reviews plugin is installed, and to enable publishing a record of the action *along* with a review (two posts in one go!).
So many ideas, so little time.
Hooray! Can't wait to get mine :)