Stallion WordPress SEO Plugin was created to enhance WordPress SEO plugins like Yoast and All In One SEO that damage a sites SEO by using nofollow.
The Stallion WordPress SEO Plugin only deals with protecting link benefit flowing through a site that other WordPress SEO plugins damages. The Yoast WordPress SEO Plugin and the All in One SEO Pack WordPress Plugin title element and meta tag features are if used correctly not SEO damaging.
You can safely use this plugin alongside other WordPress SEO plugins, for it to work correctly you should disable all nofollow and noindex features of other WordPress SEO plugins and turn off their canonical URL features. Some of the features of the Yoast WordPress SEO Plugin and the All in One SEO Pack WordPress Plugin are useful, it's the nofollow and noindex options that should be disabled.
To be honest I'm not sure. The plugin does all it's search engine optimization work behind the scenes (you don't see anything on your website as you view it), so maybe when users activate the plugin and see no obvious changes they think it's broken or maybe they assume the plugin will be like the Yoast WordPress SEO Plugin and the All In One SEO Plugin and have options for changing title tags and meta tags etc... and when they don't find those options assume it's broken.
To check the SEO plugin is working go to a relevant part of the site and view source in a browser like Firefox, for example if you have "Index First Search Result Only : Block Paged 2,3,4...**" set, do a Search on your site and go to page 2 or higher of the search result, view source and you'll find code like this:
<!-- Stallion WordPress SEO Plugin 2.1.1 by David Law http://stallion-theme.co.uk/stallion-wordpress-seo-plugin/ --> <link rel="canonical" href="http://example.com/search/keyword/" /> <!-- Stallion WordPress SEO Plugin -->
This is a canonical URL to the main search URL, if Google managed to spider a search on your site (it happens a lot) you'll find only one page is indexed. Had you set "Block All Search Results" the canonical URL code would look like this:
<!-- Stallion WordPress SEO Plugin 2.1.1 by David Law http://stallion-theme.co.uk/stallion-wordpress-seo-plugin/ --> <link rel="canonical" href="http://example.com/" /> <!-- Stallion WordPress SEO Plugin -->
And Google etc... wouldn't index any search results, any SEO benefit will be directed back to home.
Short answer is NO.
I put my time into the Stallion SEO Theme which adds 10 times more search engine optimization to a WordPress site than any combination of current WordPress plugins and themes can achieve. The Stallion theme is years ahead of anything else available because you CAN NOT add full search engine optimization to a WordPress site ONLY using a plugin, you have to SEO the theme as well. There's too much output that's part of the themes coding so the only way to fully SEO a WordPress site is via it's theme so my time goes into developing the Stallion SEO theme not stand alone WordPress SEO plugins that tinker with the SEO output.
Anything added via a plugin can also be added via a theme, the reverse is not true.
For example theme developers tend to add a sitewide H1 header including the name of the site, this is bad SEO practice, the H1 header should include the main SERP for that page which on blog posts and static pages tends to be the title of the post or page. This is very easy to achieve by editing a themes template files, almost impossible to achieve using a plugin because the H1 code is different for each theme and even if a plugin was developed to overwrite the H1 headers the CSS wouldn't match the themes CSS and the output wouldn't look right. This is just one example, there's dozens of theme elements that should be search engine optimized that plugins realistically can't touch. In comparison changing the title tag and meta tags is easy, which is why most WordPress SEO Plugins deal with the title tags and meta tags and not H1 headers, or the anchor text for Read More links or the damaging rel = nofollow attributes added by WordPress core and themes.