Ask HN: Did you build a Spotify app?

I am considering building an app for Spotify. It seems that they are very strict regarding the apps they accept (see: https://developer.spotify.com/technologies/apps/process/ ) I really like Spotify and I think I have a good idea for a useful app.

I am just wondering if there is anyone here who tried to build a Spotify app before and if yes... what was your experience with them? Did it went well? Are the guys at Spotify helpful? Is it worth it to invest a lot of time in this app's development(I'm not really looking to make any money.. I just want to make something exciting and have a good time building it, maybe learn some new things)? Are there any things I should keep in mind?

Thanks and... keep on hacking/making :D