I think it's only the small-time dealers that hate carwoo. Have a look at the comment by Scott Salzman on that forum, which fits with my own experience. I bought my car through carcostcanada.com, and my local dealer couldn't even match the no-haggle quote I got through the site (their best offer was 1k more).

The dealer I went with was a high-volume dealership in the city. Not only was their price low, but they also were very friendly and helpful. It makes sense to use sites such as this - it's a huge time saving for both the customer and the dealer.