I also bought the pixma iP5000, and haven't found any problems with pixelation.
I also readthat review that said the 5000had worse photosthan the 4000. I think that's wrong,and thatthe confusion is because the highest print quality is strangely difficult to select in the 5000 driver. You have to select "photo paper pro", then "custom" quality, then move the slider to the far right. Once you do that, the pixels become so small you need aloupeto see them. If you don't do all that, you'll get the second highest quality, which, based on photo samples I've seen in stores,is probablyequal to the 4000's highest.