I've been using the GX CS3 compressor (amazon carries it). I got this summer of 2020 and use it a lot.
Not sure what volume, I just fill the tube that connected to the rifle, one an Avenger and a Marauder FT, 4350 psi and 3000 psi respectively. I have heard that this compressor is not good for filling a SCBA tank, just due to overheating issues with the large volume.
I like it, but it's slow compared to something like a Yong Heng water cooled compressor. I'd guess that 0 to 4350 takes around six minutes, the Yong Heng will fill in about a minute. The CS3 is a lot quieter and I use the DC power connected to my car's battery sometimes which is a feature I like.
It can leak from various fittings like the release valve and overpressure relief fitting, but that's just from loosening up over time, the compressor does vibrate a lot while in use. Tighten them up and the leaks will go away. In fact, check all threaded fittings and make sure they're tight. You'll know it's leaking when it can't hit 4K psi. I have not had to replace any O-ring yet.
The water separator seems to work very well. I added a secondary separator to it just because I had one laying around. I doubt the secondary actually doing anything, I have not seen evidence that water is getting past the primary.
I disassembled it and cleaned and replaced the grease last winter. I did not notice any change or improvement in performance. The manufacturer indicates that this maintenance is not required and that could be true.
Right now I notice that a Yong Heng can be had for $200 which is an unbelievable deal. But they are higher maintenance, water cooling is slightly annoying and it has to have AC power, but they work great.
However, I still like the CS3 better due to the flexibility of DC power and low noise.