Got myself a new Canon 70-300 IS USM lens and have been unhappy at the results since. Images look soft and out of focus, esp closer to 300mm. I noticed that pics taken in Portrait orientation were coming out almost?? acceptable, so I did some test pics today. Camera on tripod, 300mm shooting to a target approx 10m away. Results speak for themselves. Lens is hopeless at Landscape orientation, better at Portrait orientation. Also did a comparison vs my Canon 18-135 lens at 135mm & shooting the 70-300 at 135mm. The 70-300 images are soft and not as crisp as the 135 lens.
The 70-300 had a problem with orientation focus when it came out back in 2006, but it was the opposite of what mine is doing.(landscape sharp, portrait out of focus)
Will be chasing Canon up this week to get it fixed, very unhappy as this is the 2nd brand new canon lens that I have had to send back for quality issues. My 18-55IS was completely soft, out of focus and after arguing with the shop assistant who was looking in the rear LCD on my 30D saying it is fine!! What the!! I had to argue my case before they sent it back. Needless to say when it came back from Canon it was razor sharp!
See comparison pic below for the 70-300, on tripod, IS Off, all same settings, left image portrait orientation, right image landscape. (Should have got the 70-200 F4 L, about the same price, but I wanted the longer reach)
It's produces pretty ordinary pics now, hope it improves greatly after Canon have fixed it.