they can pop page one if you have a keyword, they're brandable if you have a keyword
we use a couple (less than 1%) for content sites
we promote one in ppc
occassionaly we have clients acquire keywords
if the .com is gone and it's a US business we steer clients to .us, then .net then .biz
so it's a 4th choice for US businesses, as we use them
we also have clients acquire .org's and .info's for non commercial development to lead to their commercial sites, like this info sponsored by...
eventually some business will splash a bunch of money in a major branding campaign and the whole tld will pop some, but, it's a low priority tld good only for branding a keyword or doing seo keyword work
anyway, a .biz keyword will be indexed high in serp's if it's done correctly, so that alone gives it some value, but, overall the tld needs some big ad campaigns using it to make it more recognizable