The WS readygrass mat works fine. It's tough and durable and is easy to lay and conform to any layout contours. The only problem is your whole layout will look like a freshly mowed golf course.

The best basic gound cover is some cheap brown latex paint. Use a color that's a close match to the color of the soil in your area. Paint the whole layout first so you don't have any bare foam or plywood showing through. After that, get some dirt from the yard or a nearby empty lot. Sift it to get the size you want and run a magnet through it to remove any magnetic materials. Putting it in a microwave for few minutes to kill any unwanted critters is also a good idea.

You can then spread the dirt out and secure it the same way you would ballast, with a 50/50 mixture of white glue and water. Wet the dirt down first with a spray bottle filled with a 30/70 mixture of alcohol and water. Finally, use the $29 you would have spent on the grass mat to buy some WS fine truf, blended turf, and a couple of bags of clump foliage in different shades. Gets your roads and structures in place first and then fill in with the turf and clump foliage. You'll end up with a much more realistic layout for the same money you would have spent on the grass mat.