I have not actually used this app in a restaurant yet, but I did play around with it and do like the features. The reason I chose this particular tip calculator is that has the option of not tipping on tax. I strongly feel that tip should be based on the pre-tax amount as I feel its double dipping to pay tip on the after tax amount. In addition, another feature that I like is that it takes into account any coupons/discounts I used so that you are not short changing the tip for your server, since your tip should be based on the amount BEFORE the coupon/discount was applied.
KDB6151 about CheckPlease - Tip Calculator