Complimentary. Although you can probably do everything on the XZ1 you can do on the GRD, just not quite as easily or seamlessly for some things. I have the GRD3 and the X10 and I'd probably take the X10 for light travel and leave the GRD3 at home. But for a day of dedicated street shooting, I'd likely grab the GRD3 first. I can do it just as well (or badly) with the X10, but there's something about the GRD3 that seems custom made for that. Its just one of many things the X10 does well. My inclination, if I wasn't a camera freak at the point in my life, would be to have one of these compacts and a higher-end, larger sensor camera for different uses. if I was gonna choose just one, it would be the more versatile XZ1/X10/LX5 type. But there's no harm in having both. I know the GRD3/4 are great cameras, I know the X10 is a great camera, I don't personally know but fully expect that the XZ1 is also...
-Ray