While discussing gear, I went and grabbed a 6D 24-105L kit and shot it with my current primes along with the 70-300 IS USM and 100L Macro at a college graduation, and fell in love. In moving from APS-C to full-frame after using mostly full-frame lenses on the crop, I found:
-the mediocre 70-300 non-L (granted it's the best 'non-L' tele-zoom) comes alive on FF. The 6D's highly sensitive center point and higher dynamic range and lower and more pleasing noise all conspire to make this lens actually useful.
-the 100L Macro remains incredibly sharp and hand-holdable; I don't see much difference here aside from the wider full-frame field of view.
-the 40 STM becomes downright perfect- it's the ultimate 'body-cap' lens, and it turns a bulky SLR into an over-sized point-and-shoot, just with ten times the speed and quality.
-the 24 IS USM, one of Canon's new-fangled 'video-primes,' also comes into it's own, as it becomes the wide FOV lense it's meant to be. Probably the perfect 'carry-all' lens.
-the 50/1.4 and 85/1.8 both become wider and focus faster and more accurately (in the case of the 50)- the 50 became the perfect indoor portrait lens, and the 85 became the perfect outdoor portrait lens. These two are just amazing. I'll also note that I tested my spare 50/1.8 (the belligerent 'nifty-fifty') on the 6D, and found it to be quite nice. If I didn't already have the 50/1.4 as well, I might not have bought it; but as it stands, the 50/1.8 went on to help round out my sister's T1i kit with her kit 18-55 and 55-250.
-the 24-105 F4L went onto the superceded 60D for medium telephoto work in the hands of a non-photo shooter, and it did alright. I need to rag it out on the 6D before I form a real opinion of this 'kit L', but I can at least say that the build quality is deserving of the red ring when compared to the 100L Macro. It was also +$500 on the kit price, which is just too cheap to pass up, given it's $1100 retail price and the $1300-1500 price for a preferred Canon 24-70 F4L or third party 24-70 F2.8 OS/VC. The 24-105L will have to be really bad to convince me to pass it on, and I'll be comparing it on the 6D against the 15-85 IS USM on the 60D. I expect the L to win largely on account of the 6D's sensor, though the lack of range might irk me.
-the excellent 15-85 IS USM stayed in the bag- the 60D carried the effectively longer and sometimes wider 24-105L, while my sister's T1i carried the 55-250 IS, which yielded the longest effective zoom of the bunch at 400mm.
Comparing the 6D to the older 60D externally is pretty easy- on the outside they are very similar, especially in the shape and feel of the grip and the workflow, with some buttons changed or re-positioned. The ergonomic shift is gentle and quick, though the lack of built in flash is a minor annoyance and you very quickly miss the flip out LCD screen; it is THAT useful.
My first, and biggest impression of shooting the 6D is by far most the shutter- it's just so damn buttery smooth and quiet, in a way that makes you want to take pictures. The center-point AF sensitivity IS all it's cracked up to be, and the low ISO performance is incredible with a much more pleasant feel to the noise. Less noise would be better, but the noise this sensor records is at least aesthetically neutral to pleasing, unlike the harsh near-pattern mess that the sensor in the T2i/T3i/60D/7D spit out. Much less post processing needed to produce saleable art, and time is money!
My short term plans are to fully evaluate the kit lens and perform manual calibration on all of my lenses for auto-focus micro adjust. Then I need to find some hiking boots and somewhere to ruck and stalk wildlife on the east side of Dallas!
Canon 6D|24-105/4L IS USM|70-300/4-5.6 IS USM|40/2.8 STM|50/1.4 USM|85/1.8 USM|Samyang/Bower 14/2.8 Full-Manual Rectilinear Wide-angle|24/2.8 IS USM|
Canon EOS-M|11-22/4-5.6 IS STM|22/2 STM|EF-M 18-55/3.5-5.6 IS STM|