Re the slow autofocus of the lens...
Try changing the way your DSLR autofocuses - instead of using the 9 point AF function (not sure if the 450D is the same as my 40D?), change to a central single AF point. This means the lens will focus on the object in the centre of your viewfinder not an object on the periphery. This may help - I recently bought a Canon 300mm f4L lens which seemed really soft when I tested it out of the box and found that this method sorted out the problem...when focusing to infinity, the camera/lens would pick a point in the foreground to AF on making the object at infinity very soft!!!
Sorry to hear of your bad experience, especially when you've forked out a wad of cash! But, I think you should take a deep breath and give the lens another go
Hope it works out for you Chris
All the best
Doug
