Looks great. Greeble is a really useful tool. However you still need to have great painting skills like you clearly demonstrate here. Did you render a Zdepth pass to create the impression of atmospheric perspective because it looks that way?
Ya, greeble is useful, but too many people use the default settings and then declare it done. This painting does a great job of using it properly, as a basis on which to build on top of (either in 3d or 2d paint). Great work Mathieu!