I keep enough songs on my phone to be able to cast music to a BT speaker if I find myself somewhere without internet and needing music. Otherwise, I like Pandora - simple to use, playlists are already there, and everything plays Pandora.
My latest challenge: Tie my whole house audio into my HT and other misc BT speakers and be able to sync them all up and control from one easy source that my technologically challenged GF can operate without having to call me first. MusicCast from Yamaha is GREAT on the AV receiver, but from what I read the BT speakers and devices to tie it together with existing stuff is sketchy and unreliable. Sonos would work if I want to drop about $3500, but I don't. It's looking like Chromecast Audio is the way to go, but I want to wait until the Party Mode comes out first to make sure it works.. supposed to be out any day now but I can't find ANYTHING on it yet. The desire to have one app that can select any or all zones and cast music from DLNA/Plex server or stream from Pandora/Spotify all using one app that is simple to use is what drives me, but so far there is no working model. Chromecast Audio units are cheap ($35 each), use Wi-Fi instead of BT (better bandwidth and less compression), have an optical out if you really insist on having the best D/A converter, and Chromecast is integrated into hundreds of apps now. Only problem is all my devices outside my phones are Kindle Fire and I use Prime, so it doesn't make as much sense to use Chromecast for the TV's (Fire TV makes better sense). My brother is really into using his Echo, but otherwise he is ALL about Google (all his audio, video, tv is now Google based), so he is struggling because he is starting to add to Echo with lighting and the such, but it will never cast to Chrome stuff...
Technology is getting awesome but we are still just that last step away from a complete package that all works together without costing 100 times too much and without being overcomplicated. Some day...