Retina ZIIs are good for using up a spare Zorro slot :P
Seriously though, I have one, it can be used in workbench with system friendly stuff using a tool called retinaemu that shipped with it, but, it doesn't do RTG in the sense that most cards do. The general standard AFAIK for Amiga graphics is to have all the memory mapped into ZII address space, so that the RTG software can write to any location directly, however the Retina ZII only offers a 64k window, which Cybergraphx and Picasso96 aren't designed to work with. The ZIII does it the normal way, but as the name suggests, it requires a Zorro III machine.
What retinaemu does is draw the display in chip memory, and copy it over to the framebuffer RAM on the card, but, because you're limited to 2MB of chip, and need it for other things too, it's a delicate balancing act getting it all to work well, and it's slooooow. Some graphics and 3D packages can render in 24 bit to the retina, and I suppose there are image viewers and stuff for it, so it can be useful in those areas. As a general workbench card its only saving grace is that it costs less than half what a Picasso II would, but you'll absolutely need a machine with 2MB chip to make it worth even considering. Allegedly there was an experimental driver for CGX, but it's remained tantalisingly just out of reach for me, I've seen people selling retinas who claim to have it and that it works properly, but none have responded to my requests for a copy of the driver.