I've bought one of these (50mm) to go with the M6TTL I bought at the same time. I had the choice of either a s/h Summicron for a bit more money, a s/h Summilux for a lot more money, new examples of these, or the Summitar. I definitely wanted 50mm, btw, so I wasn't looking for, and don't know what was available in, other focal lengths. I definitely wanted to buy from the dealer so I could look at and feel the lenses, so buying mail-order wasn't what I wanted to do. I recognise that this limited my choice to whatever the dealer had at the time (see the choices above) but I was happy with that - the dealer concerned is a Leica Premier dealer so I was confident they'd have a reasonable selection.
Reasons for choosing the Summitar?
a) I don't need really fast lenses, so only having f2.5 isn't a problem for me.
b) I wanted something small. Looking at the specs of successive Summicron & Summilux lenses it's clear that while quality may well have improved, size and weight have also increased (probably inevitable). The Summitar is smaller than the (Leica) alternatives, and that was a plus point;
c) although smaller, it still has the feel and heft of a Leica lens - defintiely not a cardboard replica;
d) I wanted to buy something new! - especially as the camera, bought at the same time, was s/h. That may be shallow of me, but but was what I wanted. And new Summicrons and Summiluxes are out of my reach.
So that's why I bought one. I've put a couple of rolls of Velvia 100 through the camera and will get the slides later this week, but that means that at the moment I don't have any results to talk about. However, I'm confident that any problems with the images will be down to the photographer, not the glass.
In fact that's a good admission to make - I am so far away from hitting the limits that the Summitar imposes that the differences between them and equivalent limits imposed by a Summicron are irrelevant to me.