Depends. We've seen both ends of the spectrum this generation with stuff like Just Dance and recent entries in the Call of Duty series.
If you've got enormous (really enormous i.e. CoD-level) hype, then word of mouth is a non-factor since everybody playing the game on the second day of its release will naturally make everyone else want to buy it, even if the game is puke. If you get great word of mouth, but only three people initially bought the game, it usually becomes rather worthless.
Some hype and lots of positive word of mouth beat lots of hype and some positive word of mouth any day, though. It's like a Just Dance versus one of those zillions of random shooters on the HD consoles that don't do huge, but only respectable numbers.
“These are my principles; if you don’t like them, I have others.” – Groucho Marx
