Nothing helps the miles roll by quite like some upbeat tunes. And while I'm not averse to using ear buds, staying a bit more aware of one's surroundings is a good thing for all cyclists. I picked up an OT Buckshot Bluetooth speaker a few months ago after reading about it on Chris Bennett's site: triduffer.wordpress.com. I met Chris on the Tour Divide route in 2013 and really enjoy his writings.
Anyway, the speaker is great. It weighs next to nothing, is water resistant and has a nice long battery life (OT claims 20 hours). I use a mount for a Big Air from Genuine Innovations to strap the speaker to my stem or aerobars.
The speaker charges via a micro USB port so I can top it off using my Schmidt SON hub and K-Lite with its Sinewave charger on my bikepacking bike. Otherwise it lasts far longer than my legs on long rides and so works on any bike I may be riding.
The sound quality consistently impresses me. So too does the little speaker's volume. For $40, it's a great piece of kit.