About 18 months ago I went to a wedding of some friends of ours. I took the Sony NEX, as well as my Leica M3, along with a 50mm Summicron and a 90mm Summicron. The Sony photos turned out well, but truth be told, manual focusing for a wedding wasn't that fun. You can see the shots from the Sony here. Unfortunately I forgot about the film shot on the Leica and only found it recently when I got a whole lot of rolls processed.
The results from the Leica and Fuji Neopan combination were very, very good. It shows low contrast, but tons of tonality in a typical Leica way. Such a look isnt really a typical way to shoot a wedding these days, but I still love the look, and so did the bride and groom when I showed them recently.
As a wedding camera, the M3 only really works as a second or third shooter. The slow manual focusing means you have to plan your shots, and quick candids are quite difficult. The 90mm Summicron is especially difficult to use due to the low depth of field, making it really easy to miss shots.
Still, it was fun, and I plan to do it again.