Kufu - 5:07 pm on Feb 25, 2010 (gmt 0)
I've found Yelp reviews to be pretty accurate. Maybe I've been lucky.
I certainly don't think all the reviews are accurate or valid though. I caught my former employer (A well known SEO firm) putting fake good reviews on Yelp; so, of course like a good citizen of the web I reported it to Yelp. The reviews were removed.
Though I am sure there are many instances such as my example above, the reviews are usually not too far off.