Jeff Atwood, (One of the guys who made this site) wrote a great article on the importance of sitemaps.
I'm a little aggravated that we have
to set up this special file for the
Googlebot to do its job properly; it
seems to me that web crawlers should
be able to spider down our simple
paging URL scheme without me giving
them an explicit assist.
The good news is that since we set up
our sitemaps.xml, every question on
Stack Overflow is eminently findable.
But when 50% of your traffic comes
from one source, perhaps it's best not
to ask these kinds of questions.
So yeah, effective for people, or effective for google?