M's are fun to use, that's been my experience. So until you get to trust the focusing, they will slow you down if you're coming from auto focus. The quality of the images are great and they have excellent lenses.
iPhone? I'm using mine more than anything else at the moment.
If you are shooting film, the Mamiya is a really good camera. I use its cousin the Mamiya 7.
Leica does have a mystique both due to the history of how they've been used and whom have used them. and assiduously maintained by Leica's marketing.
If you are used to manual focus they won't be slower than other types of cameras in manual focus. The fit and finish on these cameras is excellent, especially older models M2, 3, 4. They are from the height of the machine age and they do feel nice in the hand. I always got along with the RF system and I think they're good general cameras that can take great pictures in just about any circumstance.