Self-reporting in all kinds of survey work is notoriously unreliable, unless very sophisticated checks and balances are put into place to correct for it.

So what does that mean to you, the tester?

While I recommend asking questions like "How difficult was it to perform this task?" and to listen for the answer, I pay MORE attention to how long or hard the user worked at the task.

For example, it once took a user 5 minutes and countless clicks to find an area on a website and when asked how difficult it was to locate the section, they replied "Very easy."