Nice-sounding, but low-gain.
Like the other ART USB interface I used to own, the USB Mix is a low-gain unit. ART just doesn't like making things too loud, it seems. To get sufficient signal strength for monitoring, you have to hook up the USB Mix to a much-higher-gain unit, in my case, a Mackie 402VLZ4. Unfortunately, that makes it impossible for me to use the ART as anything but a USB monitoring interface. I need a sub-mixer as well.
You don't have to piggyback with the Behringer Xenyx 302USB, which is sufficiently loud by itself to drive other pieces of equipment in the signal chain, sans maxed-out onboard gain settings. (BTW, the ASIO driver designed to work with the USB Mix also works seamlessly with the 302. I guess CEntrance makes the digital processors used in both mixers.)
If you just use the USB Mix with a laptop and headphones, or if you have A LOT of headroom for adjusting your desktop monitor levels, you might not notice, or care about, the ART mixer gain issue. But if signal strength is a consideration, I'd recommend getting another mixer/interface.