Great plugin, does what it says and indeed makes switching between posts a lot more convenient!
One thing though: when using it on a site that runs two languages using the WPML plugin, it shows all posts in the other language too. Now on a site with one other language this is only confusing, but on sites that have 3 or more languages, your plugin will be useless as it will become much more complicated to find the right post in the right language.
Maybe you can look into this for a future release?
4 stars now, 5 stars when you add the functionality :)