Having made some progress on my layout, I thought I'd post a few updated photos. I've finished most of my under the layout electrical work, and am moving into terrain shaping. I've been using a combination of extruded foam, cardboard web, foam core and am trying Woodland Scenics Shaper Sheets. It's still a work in progress, but here's my brief review of the shaper sheets...
I bought the 18" x 72" version from Hobby Lobby with a 40% off coupon. They're a bit salty for what you get at $30 full retail. Very easy to form and provides believable looking terrain. Easy to cut and shape. I glue into place with a just a bit of hot glue. Holds the shape very well, even without having added the plaster overlay. I'll be doing that along with adding plaster cloth and sculptamold to my entire layout in due time. I think this product can be very beneficial on a smaller layouts (4 x 8ish). On large expansive layouts, I think you'd be better off with other terrain building materials. I think it's a fine product for planning my terrain as well, as the sheet can be reshaped until it meets my approval.