I know this question has been asked plenty of times before, but from what I've read, no one has ever really got to the bottom of it.
Whilst this excellent post (How best can I structure product configuration for SEO?) finalises when attributes and categories should be used, what I've yet to balance is the implications of using layered navigation from a search engine optimisation perspective.
I've been using an extension from Amasty recently to help out with layered nav, and to be fair, it functions well. However an SEO associate has told me that he's pretty horrified at the number of unique URL's that layered navigation produces. I balanced that off by saying that I can set certain aspects of the layered nav to "No Follow", but he informs me that it makes no difference.
He advised me that really, all layered nav links should include a hash tag, so that search engines will categorically not even look at them. For example -
There is an extension that I've found that does this, but I'm not convinced. I would assume that if this was the case, everyone would use it, but it's from a pretty unknown developer.
I would be really grateful for any feedback on this.