I've listened to a lot of these tiny Bluetooth speakers, and at best they tend to sound OK, meaning, not awful. I don't think the Mini Jambox sounds great, but it does sound good for its diminutive size. I compared it with the original Jambox, and it sounds cleaner and it played as loud or even slightly louder. It offers some bass and can fill a small room with sound, but it also has its limitations, so don't expect the world from it.
If you put the speaker in the middle of a table in the middle of a room, it can end up sounding fairly thin. To get more bass out of the speaker, it helps to place it near a wall or in the corner of a room to get some reflection.
As with a lot of these little speakers, you're going to listen to some tracks and think the speaker sounds decent, and listen to others and be less wowed. The Mini is strongest in the midrange and does well with vocals and acoustical tracks; with simpler tracks that have fewer instruments playing simultaneously, it sounds best. When you get into more-complicated tracks where a lot of stuff is going on at once, things start to get mushed together and just don't sound great. There's only so much you can do with all the digital trickery (Digital Signal Processing or DSP) that goes into making as tiny a speaker as this sound respectable.
The Sallie Ford tracks I listened to sounded pretty good; her gritty voice came across clearly. It helps if you don't stray too far from the speaker -- I tended to listen from about 6-12 feet away.
However, when I went to play hip-hop and techno tracks the speaker showed its limitations. It can only play so loud and deliver so much bass. There's just not a whole lot of oomph here, though there is more oomph than you get from many tiny speakers, so it's all relative. The other good news is the speaker doesn't rattle or move around when you do play bass-heavy tracks.
Despite its limitations, I was impressed by how much sound Jawbone was able to get out of a speaker this size. I was also impressed that its engineers were able to improve on the sound of the original while shrinking the speaker down. It works well for casual listening (don't expect it to power a party). I also used it with an iPad to watch some movies and it works great for that. I had it on the nightstand next to my bed (near the iPad) and its sound is definitely a nice step up from the sound you get from the internal speakers on a tablet or smartphone.
Speakerphone performance was good -- no problems there -- and my connection when streaming from both an iPhone and an Android smartphone was mostly steady. I had a few dropouts (Bluetooth doesn't transmit very well through your body so you can sometimes end up blocking the transmission), but I found the connection pretty reliable and the range pretty good. I had no problem streaming from up to 30 feet away. And after pairing with the speaker once, as long as Bluetooth was enabled on my smartphone, my phone automatically reconnected to the speaker as soon as I turned the speaker on.
I've dinged Bluetooth speakers in the past for costing too much. For instance, I thought the Deck by Sol Republic and Motorola cost about $50 too much. And I'll say something similar about the Jambox Mini -- it's flat-out pricey at $180, especially when you can now get perfectly decent tiny Bluetooth speakers for $100 or even $50. It should cost less than $150 (and probably a bit less than that). I also think Jawbone should have included some sort of protective cover like it did with the original Jambox.
However, I do think the Mini's design is superb (though the Deck does some fun things with LEDs and colors, the Deck's design is all plastic). Also, I can't think of a micro speaker that's as small as this that sounds better. Better-sounding speakers like the UE Boom and Bose SoundLink Mini weigh more, and while they're relatively compact, they aren't nearly as lightweight or slim as the Jawbone Mini.
So what it comes down to is how much of a premium you want to pay for a very slickly designed tiny wireless speaker that sounds impressive for its size but not as good as competing products that cost slightly more (or significantly less in some cases). All I can say is that while the Mini Jambox may not sound superb, it's the best-sounding micro Bluetooth speaker I've encountered. Take that for what it's worth.