Guestbooks are pretty much a thing for personal websites, in my opinion.
With that said, they could be an enhancement to your site if you used it for customer testimonials or feedback on particular items. (Though I wouldn't refer to it as a "guestbook".) This is also achieved through the forum interaction placed on sites, which is what I am seeing more of because of the member interaction.

A warning if you do decide to use a guestbook, make sure your code is defined to reject scripts placed in the comment section! You definately don't want to risk the integrity of your site. (I only mention this as I tried to hand-code my own one time - and was strongly cautioned as a result!)