I have the Mark II and don't plan on selling it. I don't yet have the Mark III but will likely pick one up in the future.
I have no issues with the 5D Mark II's autofocus capability and isn't quite sure what all the complains regarding its AF are about. While I don't yet have the Mark III to make comparisons, everything that I have read has unanimously agreed and praised its vastly improved AF system. It's a major selling point for many that have upgraded. However, my Mark II continues to deliver beautiful, crisp images for me. I use center point focusing with back focusing to recompose and the technique is simple and natural.
Blurred images I have seen are generally a result of my incorrect settings – too low an ISO setting in dimlight which is easily fixed by pushing up the ISO to get a faster shutter speed for handholding. I rarely get blurred images. Most would fall in user's error and not in the fault of the camera. In my opinion, unless you're a professional pushing the limits shooting fast action or extreme lowlight, the Mark II is plenty of camera that can handle most situations. I've seen amazing images shot with the Mark II and even from the Rebel line and equally have seen bad images shot with the Mark III or 1D-X. In the end, I think it's really the skills of the photographer and rarely the fault of the camera. Sure, having the Mark III, a 1D-X or even a D800 certainly helps, but I also think exercising better shooting/focusing techniques will also vastly improve the final results of an image using any camera.