I read in The Princeton Review's Cracking the GMAT that it all depends. If you are a non-native English speaking, it is pretty critical. Apparently colleges and universities are the ones who asked for the writing section to be added to the GMAT. This is because non-native English speakers would pay native English speakers to write their Essays for college admittance so on paper their essays were great, but once they actually entered the university, they would have terrible writing skills.

If you are a native speaker of English it is just something that might inspire admission councilors to overlook a lacking Verbal score. So if you didn't score too hot on the Verbal section, then a 6 on the essay looks really good. If you scored a 45+ on the English they aren't going to really pay too much attention to your AWA.

If you are a native English speaker and you scored a 51Q/38V and a 4.5 on the AWA then its a bit difficult to say how the universities will perceive your scores.