I recently used it for a local restaurant I was not familiar with. Even though I was dining alone that night, I thought it might be prudent to use it in case this was a brand new restaurant and was thus packed. OpenTable couldn't find any tables for 1 at the restaurant, but as soon as I changed the number of people to 2, there was plenty of availability. A few other restaurants also appeared with availability when I changed # of diners to 2, so there may be some bias against single diners. As others have mentioned, if one particular request doesn't get you the time you want, it may be worth a few minutes to try changing some of the specifics of the request to see if the result is any better.

When I made the reservation (for 2), in the comments, I noted that there would only be one person. As it turned out, it was a small restaurant and I was the only diner there for most of my meal. The manager mentioned that they were very happy with their relationship with OpenTable. And I did mention that the restaurant appeared to be full when I tried making a reservation for 1, so I hope he worked to change that.