The Cosina 100mm is a pretty decent starter macro lens with good sharpness. You may also find the lens branded as Vivitar or Phoenix. But to get actual 1:1 magnification, it comes with a matched adapter. 1:1 isn’t always necessary, but its certainly good for anything smaller than a butterfly. While using the adapter, you loose infinity focus.
The image quality is good, but I prefer my old Minolta 50mm f2.8 1:1 Macro since the focal range is more useful for non-macro shots and it doesn’t require an adapter for 1:1 so it is much more likely to stay on my camera. Until I got the 50mm, I prefered using a Raynox macro adapter on my zoom lenses instead of the Cosina for shooting macros. Since I was using an adapter either way, I preferred the flexibility of the Raynox DCR-250 which I could use on any of my lenses.
After using the 50mm for a few years, I wish I had a macro lens with 90mm or more so I could shoot from a bit further away. Scaring bugs off isn’t usually a problem for me, but lighting them with my flash at 1:1 can be difficult since the lens is so close to the bug.