How important is the official website in evaluating a college?
The short answer on this one (unfortunately) is, "It depends."
There are a lot of great colleges out there that don't have great web sites. They aren't intuitive or navigable, they lack a lot of interactivity for you as a user, and/or they don't give a very warm, open feel to the entire school. (Alternatively, they're just bare and boring.)
This is where the importance of the campus visit comes in - and I can't stress that enough! Actually GOING to a campus, actually walking around and meeting people and experiencing those activities that you will participate in every day as a college student if you decide to attend that institution are going to be FAR more valuable than dry facts and figures (and even tons of great photos!) on a web site.
That said, however, if you aren't able to get to campus for one reason or another (maybe you're a California student with a strong interest in the University of Michigan but it's just too far to fly there and the cost is prohibitive), a good web site is the only thing that will help you gain that sense of what a college is truly all about. Most colleges are VERY mindful of this, but others seem to have missed the boat. It's up to you as the prospective student to do your homework and get to know the school using a variety of methods - and the web site is only one component of that.
The bottom line is that you really shouldn't judge a book by its cover - even if it's the only thing you have to go by.