I'd think of the 90 Macro M lens as a 90mm lens with a shorter minimum focus distance than other 90's instead of a macro lens. It's more for details - the kind you can see with your unaided eye. Pictures from it aren't magnified so much that they draw attention to themselves like photography at 1:1 or more can do.
I have one and it's great. I wanted a 90 for my M, wanted something that was small and focussed closer than 1m. It's great. The adapter lets you get a bit closer. Nice for flower type shots, etc.
If you want serious macro, just get an SLR for the reasons above.
On the other hand, if you want a bit more reach than your other RF lenses give you, want a great 90mm, and don't want to lug around an SLR in addition to your RF to achieve that, the 90mm macro is a great lens. It's a perfect travel lens - it's small and really extends your range - want to get some detail on some wood work or something like that? The 90 fits the bill. Want to see a fly's eyes? Go with the SLR.