A post of a site I like have inspired me to create a model to render text made of smoke. As always, I tried to give it a look just like you get in nature.
Despite some people draw clouds and smoke in a very similar way using other programs, usually it's not the best approach if you want a realistic result. By "realistic" I mean at least people will not have doubt about what you have drawn when they see your work. Look at my clouds text rendering bellow to see how it's different of smoke.
Above you can see a smoke word.