"They're good dogs" as they say on reddit, twitter, maybe some other website.
12" cars - just don't look right.
15" cars - better. Work on O27, look a bit silly on them. Ideal for smaller layouts.
18" cars - now we talking. Need O72 to look good.
21" cars - on an O72 curve, can see the center rail. That ruins it for me.
MTH 18" passenger cars just do it for me, long enough to get the idea, but not so long I wish I was in HO or N. Its a trade off. Then they are plastic instead of painted aluminum that can chip. It least if it gets a ding, you usually cannot see it at a distance.
I just wish we never had two, (three with LC+?, or would that be four with BlueRail?), control systems.