Thought I would chime in... As far as the mis-focus, there are so many possible things going on... First of all, the 5d2's AF isn't the best, in fact, it's just not even average... Granted, Back in the day when I used to shoot xxd cameras that had the same (similar) 5d2 AF and configuration I thought it was OK... but really the latest generation of AF in the 7d, 5d3 and 1dx are really hands down, to me, superior. They do require some relearning as they are vastly different beasts and takes practice to master, but once you do, they are hard to miss with. As far as the 1d bodies, and in general, you mention specific situations where focus was off, but what percentage of shots were off? Was it merely in specific situations on challenging lighting conditions and so on and so forth where every camera you used just bad in, or was certain cameras better than others, or...? You give a rough idea of your plight, but really if your getting 70-80% keepers and the tough situations let you down, then it may be your asking for too much... for what it's worth, 5d2 isn't the best low light camera, 1d4 have had complaints of low light tracking, the same with 1d3 and 5d classic... The 1d3 had a recall on the AF if my memory serves me right... The 1d4, in low light with single shot and manual point select, to me, should AF quite well... but I would suppose if you have it in Ai Servo in low light, It might be tough going... Also some lenses focus faster than others, which means they can lock focus faster but then if the camera for a split seconds breaks contact with the point it was focused on, could, in ai servo, quickly jump off the focus plane.. As far as my advice, if your pocketbook can afford the 1dx, get it... worst case scenario, if you dont like it, you can sell it for almost no loss in $$... If your budget is a big tight, get the 5d3... give either or both a chance for you to test, learn and relearn the system... you dont know how many threads have been started here with 5d2 and 1d series shooters moving to the 5d3 and 1dx and not being able to figure the AF out... There is a learning curve so take your time. Once mastered take them on a paid shoot and sink or swim... I think you would be blown away, but then again if you go into the shoot halfway jaded, you may leave the shoot just as critical. So anyways those are my suggestions from an honest user of Canon bodies who earns their living with photography for the last 10 years.