Very important.  Particularly if you have a large site.  We operate a large site with 100,000's of pages and as Dan said it can be difficult to maintain.  We use something called Unlimited XML Sitemap Generator which builds XML sitemaps for us automatically.  I'd highly recommend it although it takes a bit of fiddling with to get it up and running as it's software which sits on site.  We couldn't manage without it as we'd be forever on sitemaps.

We found that getting sitemaps right on a large site made a huge difference to the crawl rate that we encountered in GWT and a huge indexation to follow.

In particular check for 302's.  I made the mistake of leaving those for a while and am sure that we suffered from some loss of link equity along the way.

Hope it helps

Dawn

Hi Joel,

I echo Marek's comments.  However, I'm a great fan of making 100% sure that the bots can access everything that I want them to so if I'm ever in doubt I go with css and html combination as much as possible.  We use Ajax and jQuery totally etc only on pages which we believe are 100% to be used primarily for user experience and engagement.  E.g. When they're doing searches for specific things and the page needs to be ultra fast and efficient.  However, we also try to ensure that we have crawlable pages which output the full content of a search wherever possible so that we can get the SEO benefit too.  It also helps for when people have javascript disabled (not many granted).

I've seen so many ecommerce sites with great content but it's often got some kind of blockage that means a button has to be pressed or a form submitted to see it and if I'm not mistaken bots can't access this easily.

Hope this helps.