Apparently Yousef did respond to some of Walid's claims:http://www.goodreads.com/author_blog_posts/1246151-let-s-get-to-know-each-other-let-s-talk
The one thing that stands out to me in that entire public letter, is that Yousef champions non-denominational Christianity, which is unusual for most Christians (as they all want to clamour to an identity that fosters 'group thinking'). So maybe there is something there.
There are some places calling Mosab Yousef a fraud purely to extract money, but the problem with that theory is twofold:
#1 the sources which espouse fraud tend to be pro-islam (you know, anything that gives consideration to islam of course will slam Mosab into the ground)
#2 Mosab knows the Quran well, and because he *knows* the disparity between the Quran and the Bible means it becomes harder to be a fraud exclusively for money. Not to mention, once you 'know so much' (especially regarding the Bible), it becomes hard to lie to yourself and behave like that (re: fraud).
Peter Popoff and Benny Hinn are fraudsters because they know ZERO Bible. Plain and simple. Mosab knows both the Quran and Bible which gives the potential for merit.