I didn't! I was lucky enough to borrow this lens from the Leica store to try out. I did this, just to check if this lens would be something for me. I had recently bought the 21mm Summilux before, and was very surprised to find that I loved it, so I thought maybe I should try the 28mm Summilux as well.
For two reasons. Usually I try to stay away from 28mm. My feeling is that it's not quite wide enough, and too wide, at the same time. I also had this feeling with the 28mm Summilux, but it could be because I have become accustomed to 35mm and 24mm. Another thing, is that I could't get on board with it's size and design.
On the other hand, I have actually taken a lot of pictures that I really like with a 28mm, and which could not be reproduced by other focal lengths. But I still tend to prefer my pictures with 35mm and 24mm. Also, I find 35mm so similar to 28mm, that I usually tell myself I should just pick one of the two and forget the other. That way I can focus my efforts on one of these two focal length, and not get confused by their similarities.
Usually, I don't like lenses longer than ~4 cm, and this was also true for the 28mm Summilux. It's a heavy lens, which means that I want to hold the lens and camera by extending my left hand outwards, so I can support the weight of the camera with my palm, and support the lens with my fingertips. This way I also have my fingertips on the barrel of the lens, ready to focus. But this only works well when using live-view and when you have the camera in front of you. Also, the focus tab of the 28mm Summilux sticks uncomfortably into your fingers, when you hold the camera like this, and makes it uncomfortable to focus.
Although Leica probably wanted you to focus this camera holding the camera up to your eye, which makes sense as it's why you buy an M camera. However, when doing this I find that it becomes harder to support the weight of the camera with your left hand - which is what I want to do to make sure I'm not tilting the camera - whilst also focussing. The alternative is to support the weight of the camera and lens with your right hand, but this hand sits much farther away from the center of mass of the camera. As a result the camera, and the image, are more prone to tilting. I guess my point is that using this lens felt fidgety, but maybe a "thumbs up" could have helped. This is also why I don't usually don't like big and heavy lenses. In short, I was not a fan.
The aperture ring was extremely smooth feeling, but could be turned easily by accident. The focus on the other hand was very stiff, probably because of the floating lens element in this lens. The lens renders beautifully, like all Summilux's.
Get this lens if you if you don't care about weight and size! Otherwise the 28mm Summicron is probably a better choice. It will still give you a little shallow depth of field at 28mm, but in a much more manageable package in terms of size and weight.