WordPress.org

Why would I want to use this instead of .htaccess?

Ease of use. Redirections are automatically created when a post URL changes, and it is a lot easier to manually add redirections than to hack around a .htaccess. You also get the added benefit of being able to keep track of 404 errors.

What is the performance of this plugin?

The plugin works in a similar manner to how WordPress handles permalinks and should not result in any noticeable slowdown to your site.