I have only had good results with this plugin so far -
like other posters have said though, you have to use it correctly...
One important thing that was pointed out here though, is footprints.
One thing you should erase - open the file searchterms-tagging2.php and find the line that says "DEFINE PK_WATERMARK".
Change "<!-- SEO SearchTerms Tagging 2 Plugin -->"
to something generic like "<!-- Tags -->"
if you are not using multisite, you should change the comment to your site's main keyword:
<!-- your keyword here -->
The other footprint to remember is to never use the default values - always change them!
I use the widget, and never the end of post tags.
I add a random # of tags in each blog (not common numbers, so: 12, 13, 14, 16, 17, 18 or 19)
I turn off linking (it's just a list that doesn't link to anything)
I turn off "show search counts"
as far as I can tell, Google loves it...