It does work mate
Follow the instructions and when you are going to edit the template(s), use $threadinfo[preview] instead of $threadinfo['preview'] (note the difference).
I'm using it in the headinclude template like this:
Code:
<meta name="description" content="<if condition="$pagenumber>1"><phrase 1="$pagenumber">$vbphrase[page_x]</phrase>-</if>$threadinfo[prefix_plain_html] $threadinfo[preview] $foruminfo[title_clean]" />
Check the source here to see it working
http://www.streetcars.nl/showthread.php?t=271