So far I always used EasyGrep for replacing text in multiple files. Unfortunately it is quite slow when a project gets bigger. One thing that seems to be amazingly fast is G
cdo command has now been added! After you grep, you can use cdo to execute the given command to each term in your quickfix list:
cdo
cdo %s///cg
(Take a look at this git commit and this vim developers google group discussion for more information on cdo and the motivations behind adding it.)