Generally a nice app. What I was missing was a way to set the user globally on all sites. Instead you have to set the active user for all the sites. I don't see the usefulness in being able to set different users on different sites...
The best way would be to be able to log in, but you commented elsewhere that this is not yet supported by the API. I would see this to be the first priority when (if) it does! 

I like the rep notification, but if you have active both the main site and the meta site for one site, you get 2 rep notifications for each rep increase on the main site. The rep on the meta site does not have a rep on it's own, it just showing the rep on the main site (except SO Meta which has its own). You should not trigger for the meta site on rep changes, unless you monitor the internal rep within the meta and not the rep number shown on the user profile.

I also noticed that YouTube links in the "Musical Practice & Performance" site were replaced with large placeholders for the video, but no video. See [this answer][1].


  [1]: https://music.stackexchange.com/q/4648/249