1. Big bug in latest version (1.5.1): Can't create your own meta titles and descriptions! Supposed fix in forums doesn't work.
2. Documentation ranges from incomprehensible to incorrect.
3. Big problem with unique Page IDs. You'll probably end up with a zillion numbered URL versions of each page. And if you're using dynamic menus, you never know which of those URLs will end up as your link. This is deadly for search engine placement! It's a moving labyrinth of a site map. What a nightmare.
4. Poor support in the forums. To this point I can't even get anyone to answer a question. This is in contrast to my experience with excellent support for other extensions, such as Virtuemart and Extended Menu.
To sum up: Don't use it! Try anything else first.