It does replace the Google XML Sitemap Generator, which is nice to have just one plugin that handles that.
I haven't found it to be size and load time heavy on my site though. It does have a lot of options, but I've found that once you se it up properly for what you need it to do, you can let it be.
The original reason that I switched to Yoast, was that the All-In-One plugin was doing strange things with my categories, which I wanted to represent strongly in search. Specifically my getrightmusic.com/category/mixtapes/
Hope that helps. If you switch to Yoast and need any help setting it up, feel free to ask.
Thanks for the quick response.