My only real bugbear with App.net so far has been it's lack of a mobile-friendly interface. It got that sometime in the early hours of today (speaking from British Summertime that is). While we're going to need a native app to get push notifications, the other big deal feature for me, this is a huge step forward in the platform's usability.
I doubt we'll see any native apps on the iOS App Store for the next month, but a few beta versions are springing up, and a list of in development apps is available on github.
So far it's still a little quiet, though some of that may simply be the contrast of switching between it and Twitter throughout the day. A good number of people that I follow on twitter are now using it though, so any sense of loneliness is quickly evaporating.