I thought REG was relatively easy and I'm sure that I didn't perform well. I thought BEC was medium difficulty, and I did okay. FAR was the toughest thus far – primarily because of the amount of material covered.
As others have said, it depends on the candidate and his/her strengths/weaknesses/background. For example, with BEC I didn't really have any experience with cost accounting (my managerial accounting course counted as cost) but I have an MBA and knew the economics and finance stuff, and that's what carried me through. As for REG… yes, a tax background gives you somewhat of an advantage but I don't think it's as pronounced as some people believe; you're going to get quite a few questions regarding material that you might not even see one time during a particular tax season and the business law stuff is generally unfamiliar territory for most everyone.