I wrote this post a while back. The content can still be relevant but the information I've linked to may not be available.
The BBC reports that web users make up their minds about website quality in a fraction of a second. Blimey, not much time to make an impact!
I guess that some idea of website quality can be formed in a fraction of a second but if I am looking at a website about some new software or a web application, I like to look at a few screenshots at the very least :-)