I have used one for years. The 4060 > MPS6030 > 620 combination sounds great, in my opinion. I have also used it with MKE2s and CA11s and thought those worked well too. One nice thing is that it provides ~ 5 V plug-in power, so a lot of small mics can be powered OK without a battery box. When I really want to go minimalist, I run the MKE2s straight into the 620; it sounds good and is about as small as you can get...
I also used to own a factory 620 and had both simultaneously for a while. I never really did a formal comparison, but the Oade version was definitely quieter.