I guess not many people here have used one then?
I have one. I've had it since about October 2015. I'm not a gear snob, but I have used all sorts from low-end right up to a full-blown SSL console, so I also have real-world expectations of budget gear.
Well my opinion of this interface is that it is unbelievably good. Not just for the money either. I got it very cheap (worked out at Β£87 UK) purchased from France online. I wasn't expecting that much from the preamps, but I also believe cheap preamps sound perfectly fine. It's when you get into the top of their gain range that they start to show their shortcomings. I prefer transparent preamps anyway and don't record anything very hot.
However, I have tested these pres out and I'm amazed at how quiet they are. Another engineer friend of mine was also very impressed. It's totally plug and play too with my iMac and Protools 12.
Not a hitch at all. Very stable.
The great thing with my UMC404HD is that it was used for some ADR at my friend's studio recently and the preamp delivered the goods beautifully for a Hollywood movie that's soon to be released: Mechanic Resurrection.
So it's official - it's a pro piece of gear!
Oh yeah. It has pre-converter inserts on all 4 inputs too, which I think is a great feature. I love this thing.