I follow the startup space closely. When companies are going head to head, I love to look at traffic figures and see who is winning. Alexa while not all that accurate does show trends.
This link shows a few of the companies going after the local review market. Citysearch is the old man, Yelp, Judysbook, and Insiderpages are the up and comers. The Alexa graph shows CitySearch the far and away traffic leader. Yelp next and then it's close between Insiderpages and Judysbook.
If you do a search on Google, with site:yelp.com or any of the other domains it will say how many pages Google has indexed (this isn't completely accurate either, but good for trends) from the domain.
Interestingly, the traffic rankings in Alexa corresponds to the number of pages in Google. So the more pages you have indexed in Google, the more traffic you had in Alexa. In Yahoo and MSN, it didn't work that way. Yahoo and MSN reported more pages indexed from Judysbook than Yelp.
My conclusion is if you want to beat your competitor for traffic is to get more (legitimate) pages indexed in Google.