I am going to buck the crowd. As I mentioned, I shoot with a pair of 7D cameras and I purchased a used Canon 5D Mark-2 camera for several reasons.
First is to see if it would be a bit easier to shoot my dog portraits with a full frame camera because of the distance under which I usually shoot. I have a bit of a problem getting my larger dogs within my frame at the distance I have to shoot.
answer: yes, easier but, not greatly simplified...
Secondly is to see if the quality of full frame would be mind blowing over that of my 7D.
answer: I have not done any head to head tests but, from casual observation (using the same lens) the quality of my full frame images is not mind blowing better...
Third is that I was just darned curious about full frame and when I had the opportunity to get a minty used 5D2 at a relatively low price $900 USD, I picked one up.
What I have noticed is that the autofocus system of the 5D2 is not anywhere nearly as sophisticated for sports and moving subjects as the AF on my 7D cameras. It is more like the AF on my 40D than that of the 7D. I really feel like I am several generations behind rather than only a single generation (5D3 > 5D2) of course, I am one generation behind in my 7D setup (7D2 > 7D) but it doesn't feel like it. The autofocus is so basic and limited on the 5D2, that I will not be taking it to shoot the Blue Angels next week....
Will I keep the 5D2? I have not made up my mind yet. The jury is still out!
I am wondering if you replaced your present EFS: 18-55 f/3.5-5.6 and EFS: 55-250 f4-5.6 with better glass, might the imagery you get from your present camera improve? Better lenses would give you at least an extra stop at some focal lengths and a 17-55mm f/2.8 IS would provide two extra stops at 55mm. Even at a shorter focal length, you would effectively have at least a two stop advantage since you can effectively shoot the 17-55mm f/2.8 IS lens wide open and to get excellent image quality, you really need to stop down your kit lens a stop or two...
Of course, choosing photo equipment is always a series of choices and compromises
