This diagram, from a recent 451 Research report, is intriguing.
AWS is top of the heap, “used by the majority of enterprise IaaS customers in the study (57%) and is also cited as the most important IaaS provider by 35% of these current IaaS customers.”

But look at Rackspace, ahead of AWS on ‘promise’ and only just behind on ‘fulfilment.’ Look, too, at big — credible — players like Google, “cited by less than 10%” of respondents to 451′s questions.
It’s a survey, and survey’s should always be examined with caution… but it’s still worth considering.