I shoot weddings professionally with an M8 as my primary camera. Backup is usually another M8 or an Epson R-D1 and then I also bring a 5D which stays with an assistant unless I need it.
Overall, I love the M8 for this work but there are some caveats.
1) If the light is very low and I'm going to be up around ISO 1600 or higher, I prefer the 5D simply because the files are cleaner.
2) I sometimes have lighting sources in the frame when I'm shooting weddings by existing light (which is my norm). The IR-cut filters can tend to flare and this can be exacerbated by a lens that is not so well coated. Under these circumstances, I tend to prefer higher contrast lenses with the best possible coatings (this includes most of the recent ASPH lenses, the Zeiss ZMs, etc.) Wedding lenses I particularly like on the M8 are the 28 Summicron ASPH, 35/1.4 Lux ASPH and Leica 50/1.4 Lux ASPH *or* CV 50/1.5 Nokton. I don't filter the R-D1 (though there's some reason to) and with it I also like the CV 28/1.9 Ultron and CV 35/1.7 Ultron. I haven't yet shot a wedding with a CV 35/1.2 Nokton but that probably would work well even if filtered for the M8.
3) The method for changing ISO is a pain when one is working quickly. I made great efforts to get Leica to improve this but nothing has been done to date.
Otherwise, its a great wedding camera if you like RFs and working with primes. For flash I freehand a corded Vivitar 285 with a remote sensor on the hot shoe.
Cheers,
Sean
http://www.still-photo.net