I usually tip around 15% for good service.
The whole point behind tipping is essentially a reward for doing a good job or for going above just what's expected of you. I've been to restaurants in places where tipping isn't customary and I was amazed at how much the service seemed lacking. I'd love to say "hey, everyone should just take pride in their job and do it well, screw tips" but ultimately that incentive to get a few extra bucks does make a difference in how you're treated.
As an example, my friends and I typically go through a lot of water when we go out to eat. The speed with which our servers refill our glasses/pitcher/carafe is thus something we pay attention to, and a decent server will be noting how quickly we go through it and coming back accordingly. Both times I've been to visit my family in France, however, the water refill speed was ridiculously slow. I asked my cousins why that was and they said that there wasn't as much incentive to take care of the customers once the food was brought out. I ran into the same thing in Switzerland and Austria. Now, is it a problem? Well, no, not really - at any point we could just signal our server and then ask for the refill. But as a matter of convenience, it's a nice thing to have and something I'm going to note when contemplating a tip.
As for the smiles...well, they may be fake but I think it's better than the stoic "I don't care" expression you get at other places.