This Vivitar 100-300mm f5 Close Focus zoom was made by Tokina and is amazing. It is exceptionally sharp, handles distortion extremely well, has close focusing that is outstanding and is built like a tank. My only minor criticism is that there's focus shift when zooming but the tiny amount of patience required to reset the focus after zooming is more than offset by the quality of the photos it can deliver. The provision of an in-built lens hood is a nice touch too.
I was completely unaware of the existence of this lens until I bought the FX-70 Quartz pictured which came with a couple of ML lenses; the zoom was simply bundled with the kit. Much as I love the Yashica ML and Zeiss 100-300mm lenses, this will get more use in the future than both of them because of the added versatility of the close-focusing option which is truly exemplary. I've already grabbed another just in case some disaster befalls this one - and they're very cheap to acquire. That being said, if you have the ML 100-300mm and don't need top-notch close focusing, then there's no reason to buy this as the ML is a truly superb piece of glass.
As with all long zooms from this era, a tripod mount is a very useful accessory, especially as f5 is the widest aperture.