Hi, just thought I'd say hello on this thread, up until about 18 months ago I did wedding photography quite successfully on a part time basis for around 7 years and had two 5D mkI bodies and between them I must have taken thousands upon thousands of shots... and the cameras never let me down once. In that time I got through 3 flashguns and my 24-105 had to be repaired twice (aperture ribbon band - a common fault that costs £120 - £150 to repair) but the cameras just kept on going and going. My kit was gathering dust so I sold one body about 6 months ago and then sods law the mirror became detached in the one I kept which Canon have now repaired free of charge.
I always had mixed feelings about the 24-105 on the 5D as in the right lighting conditions it could achieve some great results, but if you presented it with any challenging conditions it would start to struggle. Don't get me wrong, its a great studio lens and can be the only lens you need to carry at a wedding which is quite liberating. But it is practical rather than 'wow' and if it is the only zoom lens you are carrying you are always going to wonder whether that aperture ribbon band thing is going to go mid-wedding..
I used to think that any shortcomings were down to the 5D mkI sensor (old technology) and nearly upgraded to the 5D mkII - but having recently bought myself a 24-70 f2.8L mkII I can see that those shortcomings were purely down to the lens (and the canon flash system which is material for another thread).
I started doing weddings with a couple of Nikon D70's but they would struggle with low light and moving over to a full frame 5D changed my whole approach. Suddenly I was taking photos at 1000 iso (or 1600 at a push) and that coupled with a couple of fast primes (50 f1.4 and 85 f1.8) really raised my game. Equipment has come on a long way since then and it seems that cameras can now almost shoot in the dark and its easy to forget just how ground-breaking the 5D mkI was and how it revolutionised wedding photography, especially in low light.
Wedding photography is partly about knowing your kit, knowing how it will react in certain circumstances, knowing how to get the best out of it, and that goes for the camera and just as much the flash. Learn these well and you will surprise yourself with what great photos you can achieve. The most difficult aspect of wedding photography is the challenging lighting conditions you encounter, very bright unforgiving sunshine alongside shade as dark as night, sometimes completely out of your control because that's basically the only place there is to do the photographs.
Every camera has its shortcomings but for me the 5D mkI is still up there alongside the best, although saying that I've just bought an unsold new boxed 1100D and put the 24-70 f2.8 mkII on that and the images are bloomin awesome.
My conclusions are that 1) full frame great for weddings in low light but you need to learn to be able to 'see' good quality light and how to use your flash when there isn't any good light available... and that 2) full frame and APS-c are both capable of producing excellent images with top quality glass on the front.