This might be a tired topic around here but it's been on my mind a lot lately so I'm gonna have at it.
Grids can actually be quite pleasing as long as you build them with some Old World ideas in mind. Really, the key to nice grids is to break them up. Work in chunks and don't rely on a fixed size. Imagine having several city planners throughout the years that each have there own vision and perfect grid layout. This way your grid has a little diversity. Change your ideas of what a perfect grid is and experiment. Even failed ideas will add some character to your city. Instead of bulldozing them or starting over, try incorporating them as a real city would.
Another good way to break up a grid is to make a bunch of small communities (residential and/or industrial) and scatter them about them map. Then link them all up with roads that go directly to each community (ignore the grid!). As each community grows, they'll bump into each other, forcing grids to adapt and co-exist. And for the perfect Old World touch, create a city center then link up all your communities in a way that "all roads lead to Rome." Then adapt your grids to these direct routes, even making your grids diagonal if need be.
And the easiest way to mix up your grids is to simply use interesting terrain and rather than overpower it, try to build around it.
