The sound quality is superb in my opinion. The system can be used for roughly 6 hours (haven't managed to max it out yet at the beach) and the volume can be turned up quite nicely. Easily one of the best devices I've used so far for the features and functionality it provides. Bose has the Soundlink systems, which are easily a few times more expensive, but offer the same type of quality (possibly a bit better even?). Spotify tracks play good and the speaker doesn't start cracking up even with high volume (unless the original audio is shit, which happens on my Genelecs too).
I basically only have my Genelec 8030s for comparison here, but the price range of these is way higher than the UE Boombox (the pair was roughly 1000 euros IIRC). For an audiophile, the audio quality is high, but not the best I've ever used. For a wireless bluetooth speaker (with the 3.5mm option still available), I'd say the device is a solid 4/5 or even 4.5/5. I'll be going out with it for a week and can provide more feedback if I remember to