'10-15% of web users use ad-blockers'

"Israeli startup ClarityRay says it’s not something looming in the misty future — it’s happening now, and it’s only going to get worse.
In a recent study, the company claims to have looked at “over 100 million impressions across several top-tier publishers in the US and Europe” finding that 9.26 percent of all impressions were blocked. The likelihood that someone is using an ad blocker varies significantly by browser — Firefox users are the most likely to use a blocker, followed by Safari (the desktop version) and then Chrome."
Source:  Data from ClarityRay, reported by TechCrunch, 18th May 2012
Note - Original PDF from ClarityRay no longer available
More discussion, examples and data in this Quora post
Note - other users in Quora put the number higher at 15% or so