<p>I've owned the 70-200 f/2.8, f/2.8 IS, and now the f/2.8 IS II.</p>
<p>The IS II lens is sharper, better contrast/color and has a better autofocus hit rate than either of the other two. It is a noticeable difference, IMHO. There's not a huge difference if you shoot a resolution chart from a tripod, but once you start shooting in the real world the differences become much more apparent. <br>
<br />If you're looking at buying a used lens (what I would recommend), I think the improvements are well worth the $500 or so price difference. </p>