The second you turn on the product, all knowledge that google and other search engines have of your site is essentially gone. All URLs that are correct the moment before you turn on the product will get 404s the moment you enable SEF URLs. The tool a "404 request manager" to show you all the 404s that your site is now generating, and allows you to do 301 redirects from each old page to each new page. But:
1. You have to do it one URL at a time. Each URL takes three clicks and then you have to wait for that one to complete before you can go on to another. If you have 100s of pages of content, you're going to be there a while.
2. The documentation mentions that you need to watch the 404 request manager, but it says that you should only be getting those if you have "bad links" or "update links," making it seem like it's your fault that you have them. It says nothing about all of the links in Google that are now bad because you just changed all your URLs.
They know each old page; they know each new page. What's stopping them from putting in a 301 redirect from each old one to each new one?
FWIW, I confirmed this behavior via support:
Thanks for you report. We have modified documentation to make the fact that URLs are modified - but this is kind of the point of getting a SEF extension! - clearer.
I do want to point out though that contrary to what you say, we have no idea what the old urls were, as they are made on the fly by Joomla and not store anywhere.
Whenever possible, we do try to auto-redirect from old to new, and that's enabled by default for sites previously using non-sef urls.
For sites running Joomla SEF urls, that's much more difficult because of high risks of "collision" between and is disabled by default, but can be enabled in configuration.
This extension is a prime example of how to make money with open-source software. This version is free, and it's awesome. Then he offers an even fancier version with even fancier-looking menus for $50. For what I'm going to do, though, I'm going to stick with the free one.
I do have one minor complaint. If you enable the cache option, there's a bug in the code that the author is aware of and has a fix for. See this web page: http://www.swmenupro.com/index.php?option=com_smf&Itemid=302&topic=4670.0
But he said he'd fix that in the core code back in April, and it's now July. It's a one character fix. Either turn off caching or add a digit in the code; it's up to you.