The alexa toolbar has traffic rankings on it. They are quite revealing about your competitors. There is even a comparison chart you can look at.
You have to use alexa, quantcast and ranking.com altogether as each has different rankings and findings. I would take all three, get the average and go from there.
If they are using adbrite for advertising their site the best way to do it is click on the "your ad here" part, you should be able to get an average of how many uniques they get and how many impressions they get.