While diffraction is a very real phenomenon, opinions about the effects on an image can vary widely. And subject matter can also affect how much diffraction will matter in an image.
For example landscape photographers will generally try to avoid diffraction, yet macro photographers will generally accept a certain amount of diffraction. Reason it is accepted in macro photography is that the added elements through a greater DOF outweigh the small amount of loss due to diffraction.
As I stated earlier, Cambridge in Colour has a good tutorial on diffraction. In Part 1 if you scroll down to ‘What it Looks Like’ they provide a sample where you can roll over the f-number to see what the image looks like with and without diffraction.
Also a Google of ‘diffraction sample images’ turned up this page at DPreview where someone put up examples. But if you notice all the samples are extreme crops of small sections of the capture.
So yes diffraction is real, and the smaller the pixel size the quicker the onset. How much that will affect your images is up to you.