Router planes have been around for years. Many companies made/make them to include Stanley, Sargent, and even current versions by Lie-Nielsen. As far as utility goes, I think router planes are really under rated and under used in woodworking. Prior to the introduction of the electric routers that we are all very familiar with, the router plane was, and still is, extremely adept at smoothing the bottoms of dados, grooves, and other surfaces that run parallel to the face of a board. As with any plane or cutting tool, a really sharp iron is key to obtaining great results.
One of my favorite router planes is the tiny Stanley #271. It was manufactured by Stanley between 1926 and 1973. It was only three inches long and supplied with a 1/4" wide cutting iron. As you will see, the plane could be used in an open or closed throat application. It's a fantastic tool for cleaning out the bottoms of tight dados, and can really get into the corners of stopped dados and grooves. I've even seen the #271 used for cleaning out the background on small relief carvings. Most of the Stanley #271 planes that I've seen are usually nickel plated. However, during WWII, Stanley stopped using nickel plating because nickel was being used in the war effort. As a result, many of Stanley's nickel plated planes, to include its routers, were finished with japanning during the war. These japanned planes are significantly more rare than the nickel plated versions. There's absolutely no difference in how the japanned and nickel plated planes function. User quality, nickel plated versions, are easily found at online auctions, flea markets, etc., for reasonable prices. The #271 is certainly a handy little plane that I'd recommend.
Below I've included a few pictures of the Stanley #271 (WWII japanned version). This particular plane was still in its original box and complete with an original Stanley Router Plane pamphlet when I found it.
Jim C.