Please read for an updated status on RailsCasts:
Learn more or hide this
GitHub User: johncblandii
SEO is one of the biggest problems with single page apps and goes back to Flash days as well. Even angularjs.org has bad SEO. They are working on solutions though but this is a global SE issue, not Angular specific.
It is definitely something to consider when choosing the tech for your project.
Here is more information on validation: http://docs.angularjs.org/guide/forms.
I HIGHLY suggest validating on the client and backend to cover your bases.
It isn't just for single page apps. You can put ng-app anywhere so one single widget on the page could use Angular.
If your needs are grand (one-page app), it works. If you just need a small widget/form, it works.