Great but social aspect needs work
The iPhone app is the best thing ever. Its constantly being updated and is your gateway to finding wonderful places in your own backyard or where you are travelling to. I use it constantly and find it incredibly useful.
The iPad app is the most frustrating thing ever if you wish to engage in the social aspects of Yelp. Basic functionality such as the talk section, news feed, messaging, and viewing the details of a persons profile are all missing (and have been for years).
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Yelp: Food, Delivery & Reviews