It's an invaluable resource. In the past week I realized it is quite likely that a site I used to work at which hosts a lot of my portfolio might disappear or be heavily amended. So I located and indexed a complete list of my articles there -- and I was even able to click a button and create an archive for the few pages that IA hadn't bothered to index (I wish I had known about this before, since it was a shock to find that IA can be quite selective, and to find a page you were hoping was there simply isn't, and is now irretrievable).
But as one other commenter here has mentioned, you're only a robots.txt amend away from the oblivion that the entire IMDB comments section fell into [1], so a good archiving system is essential. I use (no affiliation)Save Page WE on Waterfox:
https://addons.mozilla.org/en-US/firefox/addon/save-page-we/