24-85 lens on rebel XT

Hi,
I've bought a 24-85 usm lens from ebay and I'm quite disappointed because
the kit lens outperforms it at any focal length/aperture. Even auto focus
isn't faster at all than the kit lens and is quite inaccurate. Otherwise,
the lens is clean and with no major wear signs.
I've already got burned by the customs with almost 50% of what I paid on
the lens and returning (if it is possible at all) might not be an option
if the seller will retain his customs duties from initial price.
What do you advice me to do? How expensive would be to send the lens to
canon service centre? I'm just afraid that I'll end up with more than the
price for a new lens and much more trouble.

Thanks