Hi M'ate
I've been using exclusively a single M8 body plus 3 or 4 lenses (CV 15, Nokton 35/1.2, Leica PA 90 cron & 135 elmar) to shoot weddings professionally for the past year. Back up is a Leica R8 film body & two lenses which has never come out of the bag...IOW, no reliability probs at all so far with the M8 (touching wood, obviously).
I'm using a Metz 54 flash (mentioned previously) which felt horribly unbalanced & heavy at first but after a while I got used to it. My advice would be to forget TTL and use it in auto mode (you need to manually change aperture setting on the flash but don't have the annoying, moment-spoiling pre-flash to deal with).
I generally pack 3 batteries for the M8 but turn off auto-review so I usually get about 350-400 shots per battery, even with a little chimping. Taking about 80-90 pics per hour, so about 1000 for a full days coverage.
Personally, I haven't found the lens changing aspect a problem at weddings as I always used to do it shooting film anyway...I guess the trick is, and you probably know this, trying to anticipate events and then working quickly & within the time you've got.
Finally, I find that the 'solidness' of the M8 helps in dealing with those pesky photography-enthusiast guests who always seem to be getting in the way...a simple cry of "your going home in a red cross ambulance" followed by a crack to the head from the Leica swung rapidly from the end of it's strap, soon puts things in order...A cheaper, plastic camera wouldn't be nearly as effective : )
Seriously tho', the M8 is fabulous for weddings, & if you're concerned about high ISO noise, use some fast lenses and be careful not to underexpose at 640 & 1250 as mentioned many times already, and then there's always Noise Ninja...Here is just some of my stuff if you're interested @
www.weddingpaparazzi.com.au
Hope this helps.
Cheers
Evad