I attended a con­fer­ence fea­tur­ing Ken Schwaber in May where in answer to my ques­tion he offered that most Scrum projects and imple­men­ta­tions fail.  Back in 2008 he was more explicit in an inter­view say­ing 75% fail.  Yes, 75% fail:

“I esti­mate that 75% of those orga­ni­za­tions using Scrum will not suc­ceed in get­ting the ben­e­fits that they hope for from it.”–Ken Schwaber

Com­ing from the co-developer of Scrum and a sig­na­tor of Agile, that’s a rather damn­ing statement.

Source: http://www.agilecollab.com/interview-with-ken-schwaber