I'm a big believer in paying for what I get. If the service is slow, and the server acts like he/she is doing me a favor even waiting on my table, I've been known to leave as little as 25 cents. I purposely leave a quarter, rather than nothing, to insure that the server knows I didn't forget, the quarter represents what I think their service was worth.
On the other hand, if the server is cheerful which adds to the enjoyment of my meal, and his/her service was excellent, I'll leave as much as 30%.
In fact, I wish we could go back to how the TIP (To Insure Performance), was originally administered.
A person when first sitting down to a table would put money in the middle of the table so the server could clearly see it. As the meal progressed, the patron would add/remove/leave the money, so the server could see if the patron was satisfied or not. The server had a running status on his/her performance.