Late to the party, but my son and I have enjoyed a number of these classic systems.
American Skyline was a recent addition from a friend that my son is playing with now that will probably use to setup temporary structures on our HO layout. I like the slightly gothic look that echos quite a few buildings here in Chicago.
Guideancetown USA is another HO-scale building system. I have a kit that I got (It was old then) as a kid from the resale shop. Recently I added another box of parts and my son has really enjoyed building and putting on our in-progress layout. It's a bit limited by having only one side with windows/doors, but you can quickly build masonry hi-rise buildings with it.
For those interested in Girder and Panel, they actually made a reappearance for a couple years "Power Train" and "Power City" system. I think they were phased out, but might come up on ebay, etc. The girders were red instead of the traditional blue. I reviewed a set for my wargaming blog a while back. I finally found some with the panels. This year I plan for them to make an appearance on my O-27 Christmas Layout. They are a bit frustrating to assemble, but look pretty cool with or without the panels.
