I did try those out in a music store while shopping for some lightweight powered speakers for my keyboards, to use at a low-volume gig. They were cheap, small and light and had a built-in mixer, my primary prerequisites. Unfortunately, they distorted badly and were very weak in the low end.
But that was with piano. I think they'd do fine for a solo singer/acoustic act at low volume, like a dinner set in a restaurant. But definitely not for keyboards at any volume, and I'd be reluctant to run more than one instrument and/or singer through it at the same time.