Imagine getting in your car, your phone connects via Bluetooth and you want to open Spotify to play some music. So you click on the app icon, the app opens and for a while you stare at a black screen. If you're lucky eventually the GUI starts to build up. That is if you are in a 4G network at the very minimum. If you're not, you may very well be notified that you're offline, rgeradless whether you are or not because the app fails to communicate with the Spotify servers. I wonder why I'd have to connect to the servers in the first place when all I want to do is play music that is locally saved to my phone. 
Regardless, I go to my libary, which again may or may not be loading at this point and try to enter a playlist, which -you guessed- may or may not be loading again, eventhough it is entirely saved to my SD card. So in a nutshell, the app is painfully slow and sometimes doesn't boot up at all. It is also dependend on your connection to function halfway decent, even though you're not trying to access Online content. 
Eventually you may have gotten past this and you are now actually enjoying your favorite song while driving somewhere. That is until the app freezes out of nowhere and no longer responds to any input. Even closing the app completely and opening it again will not help at this point. This has happens to me often enough to write this rather frustrated entry. Spotify may have the largest libary, but the app has been a pain to work with and the algorithms fail almost entirely to predict which kind of music I could enjoy. But maybe that's just my specific taste in music. 
Regardless, I'm downloading Google Play Music now and if they have the music I like, I'll switch back to them. At least their app works.