We use all O-27 profile on the layout. We have O-42 which was made with K-line on the main outer loop. We also use a lot of O-27 and O-34 curves and switches on the inside industrial and switching areas mainly old Marx switches as they are dead simple and reliable, and the boys love flipping them manually. Here is a video of our Railking YB6 backing some passenger cars into one of the industrial areas. We've updated it a little since this was taken, but still more or less the same. It backs up from an O-42 K-line manual switch/curve into it.
We've also added some K-line Super K straights on the main line since that video was taken. (You can see in the picture below) I like the look of the more sparse stuff on the sidings as @Trinity River Bottoms Boomer was speaking about. Now, this is a high end layout by any stretch but it is good for my two boys.
One last picture is the yard area, again, mostly O-27 old Marx track/switches and some Lionel. I have two more of the K-line O42 switches in the yard, and two Marx O34 switches that lead into it.
