I've listened to the Flex as both a background speaker and as a desktop model for some time, and I haven't found a genre it doesn't like. Whether it's streaming the excellent internet radio station Radio Paradise or hi-res rock tracks from my home server, the Flex always sounded balanced and, I want to say, "cheerful." Vocals are crisp, and bass is full if not all that extended.
I compared the speaker against competitive the Play:1 ($185 at Amazon) and found the Sonos offered a big sound with Alt-J's "3WW" while also able to pick out details such as the crisp shaker egg and handclaps. In comparison, the Bluesound was more constrained to the physical cabinet, details weren't as present and the result didn't feel as alive. When compared against the Denon HOES 1 the two exhibited very similar sound signatures with this song -- both a little closed in but still very enjoyable. In this three-way battle I'd give the sonic edge to the Sonos -- a larger, louder sound is better for a speaker of this kind.
Depending on where you place the speaker, it's worth experimenting with removing the cover and the foam bung. Sitting on my desk with several feet between the speaker and the wall, it sounded best with both removed, aka Ugly Mode. The sound was richer with a fuller bass, and was less "cupped" sounding in the midrange.
If you put it close to a wall, however, you will want the bung back in, and may also need to use the EQ in the app to lessen any boom. I originally had the port unblocked during one song, but encountered some extreme chuffing or port noise so blocked it up again. The EQ also added a few dB to the Treble control for a little bit of extra zing.
At this level there are plenty of Wi-Fi speakers vying for your attention, and perhaps Bluesound isn't the first you'd think of. But it is a very Flex-ible unit and is capable of very creditable sound. If you had to choose between this and the portable HEOS 1 player it's a very tight race, however the Denon has a slight edge due to its cheaper price and more robust build. If you're an audiophile though, with an interest in hi-res, MQA and Roon, then this is probably the compact speaker to get.