I'm pretty forgiving in my reviews of Bluetooth speakers because most of them are designed for more casual rather than critical listening. But when you get into the $300 range, I start being more critical.
The 855s sounds better than most of the very compact "mini" Bluetooth speakers. It plays loud, has some nice kick to its bass, and holds together pretty well at higher volumes so long as you don't feed it tracks that have a lot instruments playing at the same time.
For instance, it had some trouble with Pink Floyd's "Interstellar Overdrive," distorting badly at higher volumes. It's a hard track for smaller speakers to handle and while the 855s isn't a big speaker, it's fairly beefy for a portable Bluetooth speaker, and I expected a little more out of it.
With other less demanding tracks it came across much better. Dave Matthews Band's "You & Me" and John Legend's "All of Me" sounded clean. Genesis' "Tonight, Tonight, Tonight" had some decent kick to it. It does pretty well with techno and hip hop, though I think you'll find it sounds a little better outdoors than indoors (the BRV-X actually has separate indoor and outdoor modes).
It's worth mentioning that this is considered a stereo speaker but because the drivers are so close together you get little to no stereo separation. There is an option to wirelessly join two of these speakers together to form a left/right pair for true stereo sound, but I'm not sure how many people want to spend $600 to do that.
As I said, the 855s does offer very good battery life -- a whopping 20 hours at moderate volume levels (mileage does vary according to volume level and you may be able to do even better than that). So you you should have plenty of battery to get through a day and even charge your phone in the process via the USB charging port.
Speakerphone performance was decent but not exceptional (it helps to stand within a few feet of the speaker rather than talk from the other side of the room).
In all, I liked this Braven and think it's a solid choice if you're looking for a little beefier, rugged portable wireless speaker that plays loud and features excellent battery life. That said, I prefer the $200 UE Boom because it's more compact and easier to tote around. I also like the TDK Life on Record A33, which can be had online for around $150 and is a better overall value.