Rare? You can still buy it at several shops, …
This is a very good SATA card. I get 95 MB/s throughput inside my G4s at the PCI Bus with it. But beware it needs at least a PCI 2.2 Bus, that means it will work at Grey G3s and up without any problems. But the older PCI Macs like the 9600 with an PCI 2.0 Bus will not work (well). I needed several months to figure that out back than in 2006 when I got mine, as it is working inside the PCI 2.0 Busses but with many problems and system warning all the time about datalosses etc.
So it is perfect for G3s and G4s, and quite cheap. But if you like to upgrade an older Mac, get some Sonnet card.