Gif from - http://www.runningwithspoons.com/2013/02/05/the-truth-hurts/
Firstly, being nice involves kindness and sympathy for the person you are speaking with or going to talk about. Putting others down never uplifts you, even if you feel like it at the time. I've had occasion where I thought that the best comeback in the world would make me feel so much better. I could just put that exacerbating know-it-all or that smug, cloying skinny-minnie in their place. What I get when I feel this way is no less than a internal wobbling of my conscience. Folks are always at their best. We have no way to judge who they are because they have had their own experiences and their own reaction to things.
This is not to say that being a doormat is the way to go. Keep yourself in a good spot by steering clear of haters. Float above the filth and smog of those who decide to belch out hate. If you can't say something nice, don't say nothin' at all. And maybe, get the hell out of that conversation if you can manage it too.
Second, honesty is still the way to approach things. Do not be nice just because. Don't tell someone they have a lovely singing voice if you are considering whether or not you can hide your set of earplugs under your hair. Tell them that you what you enjoy about them instead and then move on.
Being nice really is the way to go most of the time. Make that your first option in situations. Trust me, it feels better than that perfect zinger. :)