You should really check out the photography on the net forums. If that's the lens I'm thinking of, it's of mediocre quality and you're better off spending the money on something else. Then again, purchasing a high quality telephoto lens can be expensive. If you really want a lens to be proud of, either save your money or just outright buy one of these:
http://www.bhphotovideo.com/bnh/con...lSearch=yes&O=RootPage.jsp&A=search&Q=*&bhs=t
$539 after instant rebate. Used will probably be a little cheaper. It's a "pro" quality lens, has an excellent build, and despite the fact that it's 100mm shorter on the long side it's probably one of the sharpest zooms made for a Canon camera. I believe even the 70-200 f/2.8L (non IS) has to be stepped down a spot or two to match it, it's just a constant aperture of 2.8 which makes it more expensive and desired.
And you can always pick up a 1.4x tc for under $300 that will extend the range.
I don't know if you've looked at this lens before, but don't you hate it when some one shows you something like this.
;D