I started using them a few months ago. So far, I've done about 4 orders with them, each one with increased complexity, though all 2 layers only, and without any issues.
I can only compare to Seed, which I used before. The copper layers is just as fine as Seed. In the solder mask, they appear to have some kind of issue with cleaning, as I always find some "fibers". Nothing serious, but it shows. As for the silkscreen, Seed is definitively superior on this one, though Pcbway is more than adequate for me.
In case you were wondering, I was a happy customer from Seed. I only stopped using them because they repeatedly sent my order using Fedex instead of DHL as I had selected (DHL doesn't charge me any clearance fees, while Fedex charges about $90 per shipment, so it was hitting me hard). Even further, they told that "DHL doesn't ship to your country", even though they had shipped to me using DHL. Pure bullshit.
At least PCBway ships with DHL, which is fine for me. I would prefer to save a few bucks with Hong Kong Post or similar, but our local post office is unpredictable: it can take anywhere from 2 weeks to 4 months. I cannot afford that.