How important is the official website in evaluating a college?
I'll answer this one like my Jewish aunt used to love to answer questions...with another question!: If you're trying to figure out whether or not to date someone, how important is their Facebook profile? Chances are it's not that important, because people carefully control what comes through on their FB page. They're not going to list out all their flaws and bad habits, only the stuff they want you to know about them.
Ditto for college websites. They're only going to showcase their positive attributes, and try to hide everything else.
That doesn't mean you should ignore college websites altogether--you should definitely poke around and see what they have to say for themselves. And you can learn a lot about a school by seeing what THEY think are their best attributes (which might be very different from what YOU consider to be important attributes). For example, if you're an artsy-fartsy type (like I am) and the college website keeps touting its amazing sports teams and 32 intermural sports clubs and new soccer field while saying virtually nothing about its arts facilities, well then the college might be an amazing school...just not for you.
So while you should definitely check out the college's official website, you should also consult outside sources, from school rankings guides to alumni to current students to knowledgable educational consultants. Combined with a campus tour, they'll be your best guides.