I wonder what the chances are of these new gTLDs being as successful as, say, .biz or .mobi? In other words, not at all successful.

One reason to be optimistic about the failure of gTLDs is recognition. I think people understand that something ending in .com or .net or a ccTDL is an 'internet address', but how do you indicate this with a gTLD like 'book' or 'blog' where there is no visual hint to indicate that it is a domain?