After users had worked with a number of sites, they were asked which sites they "like." What they found was that there was a strong correlation between the user's success with a site and their "liking" of it, though this wasn't in perfect lock-step.
When asked about the kind of site that they like, users uniformly replied that they like sites that had content that was interesting to them and relevant to their goals. When asked about the kind of site that they dislike, the reasons cited is typically related to a specific usability problem. It is expected that these two qualities together form the user's overall impression of a site.
This implies that users "like" sites that are easy to use - but my sense is that it also reveals a bias in the study: a user will be less critical of a site, and more likely to work to overcome barriers to usability, if he has a favorable impression for entirely unrelated reasons (e.g., sympathy for the company/organization/person who operates the site, or aesthetic appreciation to the visual design, etc.)