YES and NO!
They can give you total control over how your URLs are generated. BUT, they can also clear their list of URLs and rebuild them in a completely different way.
I've seen sites with thousands of 404 errors and htaccess files that are 10MB in size. That's the amount of data in most YouTube videos.
So the question is are they worth it? I strongly believe NOT, I don't believe SEF URLs are that important any more and the issues that third party Joomla SEF extensions cause are simply not worth the minimal gain you might benefit from having them.
