Ok in the last couple weeks I've been around the WP plugin block (at least 3x). All the other share buttons are broken, have limited options, bad graphics, or spam-like setting pages. This one has a simple and clean interface, essential options and helpful/concise instructions on the right margin. Also, it looks like the changelog shows a commitment from the developer to maintain a high degree of excellence.
The best feature for me? the ability to use shortcodes to turn off the buttons on posts/pages where it's not wanted. The icing on the cake is that you can turn it off per category.