Well, I haven't been active on DA for about 3 years, but I'm moving back into photography, soon full-time, so I figured this couldn't hurt.
This past weekend I shot a wedding with a full kit of top-end Canon gear - about $16,000 worth of stuff. I have some observations I made that day, and as I've been going through the post on the images.
1DS Mark III
15mm 2.8 Fisheye
70-200mm 2.8L IS
85mm 1.2L II
Compact Prophoto 600 kit
First - the 1DS III is everything you've heard, and more. As a piece of photo-making equipment, it is unparalleled. It handles absolutely perfectly - every button is where it needs to be, every function can be changed to suit your shooting style, it's quick, and it's responsive. As to the image quality - superb. WITH THE RIGHT GLASS.
I was concerned going into this shoot how the glass would hold up to 21mp of resolution. Sharpness and chromatic aberration were my two primary concerns. Except for the 17-40 (a 16-35mm 2.8L II wasn't available), everything held up fairly well. Sharpness on the 70-200 stopped down a stop or two was quite good, same with the 24-70. Wide-open however, not so much. And wide-open is the way you need to be shooting for most weddings, both for depth of field, and just to get the light you need. (I DESPISE artificial lighting). Chromatic aberration was not much of an issue at all, but we didn't do much outdoor shooting.
I had never shot with a prime before - and in a way it was a very freeing experience. As opposed to just standing in one spot clicking away, you're forced to move around more, observe things from different angles. However, the quality of the images I got off the 85mm was what really astounded me. The amount of light you get at 1.2 is nothing short of amazing - I was shooting at ISO 400, with shutter speeds of 1/125 or so, in a church with natural lighting. Even wide open, it's sharp edge to edge. And the depth of field at 1.2 is razor thin. If you're not careful, your subject's nose might be in focus, but not his or her eyes. It's definitely a double-edged sword - beautiful if you use it right, but very easy to get burned with.
Zooms are obviously convenient, and even necessary in certain circumstances. However, as I build up a new kit, I'm definitely going to be investing in quite a few of Canon's high-end primes.
That being said, I would LOVE IT if Canon would spend less time in the megapixel race, and more time developing some new technologies for lenses. What's the point of 21 megapixels if I have to spend $7,000 on a few prime lenses just to get decent quality images?