I bought this for a 333MHz Grape iMac G3 (tray, not slot). Setup ended up being nontrivial and required a lot of extra removal and reinstallation of the SSD. I'm not sure if the problems I was having were due to other hardware on the machine - I recently inherited it from a friend, and it still had its original 6GB hard drive. Everything worked great before I did this upgrade.
I ended up having to put the SSD in an external enclosure and install OS 9 to it from another Mac, then reinstall it in the iMac. Somehow, through all of the extra work I had to do, the CD-ROM on the iMac has stopped functioning entirely. It powers on but can't be seen by the OS.
I think if this drive is listed as supporting older Macs, maybe OWC can offer an option that pre-initializes the disk so it can be used immediately. I think part of my problem was that the drive wasn't formatted, and my iMac can't boot off a partition larger than 8GB. With the uninitialized, unformatted drive installed, I couldn't even get the CD-ROM to boot - I just got a floppy disk with a flashing question mark after the boot chime. Once I'd formatted the SSD with an 8GB OS partition, it was bootable, but since then, the CD-ROM has been invisible to OS 9, and I can't seem to boot off a CD at all now. The drive boots if set to slave or master, but not if set to cable select (saw that in another review, that seems to only apply to slot loading fruit iMac G3s).
TL;DR preconfigure the drive for old Macs as an option on this product. It would save a lot of headaches, I think. Either that or my iMac is possessed.
Also, I REALLY would have liked to see a custom power cable between the adapter and the iMac. Now I have a big wad of unnecessary matrox wiring stuffed behind the drive. Definitely something to consider adding. I'd also recommend putting a panel behind the through-hole solder points for the IDE connector, so that it's easier to get enough leverage to plug/unplug the cable without risking bending any pins. Those solder points are stabby and hurt.