Heeeeelpp!!!!

hy guys,
i own a canon 400D and i recently bought a lens canon 70-300mm F4-5.6 IS USM for a safari.
but unfortunately the picture i took at 300mm are not sharp at all and if i magnify than is a really nightmare. i had a sack of beans on top of the roof of the vehicle , so the camera was really steady. i used the camera on automatic. what will be the problem? why does it happen?it shouldn't with the price i've paied the lens.
my friend who own a nikon with the nikon lens 70 -300 bla bla took much more sharper shot.he could even magnify the picture whitout ruin the sharpness of it.
any answear for that?

hope i don't need to throw my camera in the bin. :((((

There is a number of reasons why this may be happening.
As you say, you had the camera on a beanbag, but was it held on it? Was it placed and fired by self timer etc? Shutter slap can induce camera movement at longer focal lengths.

70-300mm's are normally softer at the long end and you may have a bad one. Is it ok at the shorter end?
You could take it back to where you bought it and ask to try another copy.

Ian