I bought one based on the ASR review and great performance back on March 19th. Week of July 4th it started sometimes stuttering/disconnecting from USB audio. I'd unplug USB cable, plug back in, or switch bitrate/mode and it would work again. Thought maybe my cable was bad and ordered a replacement USB cable. By July 18th the unit didn't work properly at all anymore, could occasionally get like 2 seconds of audio out of it. New cable didn't help. Finally came on here to check, saw this comment, checked Amazon reviews, all the failures sounded identical to mine. I couldn't get any support response.
I mostly used the unit with a solid state amp, but I also flip to a tube amp sometimes, and one of the disturbing things about the failure mode of the SMSL 500 MKII was that it started making one of the output tubes redplate. I thought the tube had failed/was failing, or the amp was bad, but flipping tubes around didn't help at all. Ultimately replacing the failed SMSL 500 MKII solved the issue and the amp is back to performing as well as it can. So while I didn't notice anything like that into my solid state amps I assume the SMSL was actually outputting some sort of bad inaudible distorted signal or bad voltage that could potentially damage an amp.
Bought a Schiit Modius E instead, I wish ASR had already reviewed it, but the prior Modius had good performance and the ESS chipset based one seems like it should too. Sounds good so far but I haven't done any measurements as I don't really have a setup to do so.
So long story short, I loved the SMSL 500 MKII, sounded great and measured great. I didn't really use the headphone out, MQA, or Bluetooth but liked having the various features. At the end of the day though I'd rather have a DAC with most likely good enough to not be audibly different performance like the Schiit Modius E but from an American company with good support where I'm confident I won't have a expensive piece of trash in a few months. Lesson learned I guess.
Edit: Also worth noting, Bluetooth still works fine, and I presume other inputs do. It seems like specifically USB that died on the SMSL. I have tried it with multiple PCs, and two separate Macbooks with the same broken results since it failed. So I'm reasonably confident the issue is not the PC or the USB cable or anything but the SMSL itself dying at this point. The number of Amazon reviews saying the same thing would for sure give me pause if I was buying currently.