Well, so I've been shooting with the 90mm Elmarit-M for a while now, and I really like this focal length. The copy I have has a slight front-focus which can cause some troubles wide-open and at minimum focus distance (MFD). That's why I usually shoot with f/4 with it, in which case the front-focus problem goes away. In doing so, I've found that f/4 is quite enough for a 90mm lens to get the bokehs I wanted, with speed being the only compromise.
So... I've been looking around other options and was attracted to this 90mm Macro-Elmar-M. It's been touted as being a worthy competitor to the 90mm APO Summicron in terms of image quality. But I think the most desired feature of this lens is that it is collapsible, which means the lens can retract into a smaller size when not being used. Lens performance and compacted size comes with a premium in the Leica world, I think, and when collapsed, it feels smaller than the 50'Lux, making it easy to carry around and saves space in the bag. The hood can be attached up-side-down to get out of the way and protect the lens body. I feel a lot of thoughts have gone into the design of this lens and will make it a great travel companion.