I think I dare say they are good. They were advanced for their time. I think this is a model developed after Singer bought a sewing machine factory in Germany before WWII (from the top om my mind, I can't remember exactly so it needs to be looked up). The only thing against them is the special needle, 206x13. It's available in only a couple of sizes. Some have these machines set for regular 130 needles. There's a bit of info on it
here among related models.
I live in Norway, and I have seen two the last 3 years. Most old Singers here were made at the Kilbowie factory in Scotland. They are not very rare, but not very common either. 319 and 320 are a bit more common.