While I haven't found running a steam locomotive in TrainZ or Rail Simulator to be as much fun as on my layout, it certainly is a different experience...very enjoyable.
I would think, having a couple of them, that the simulators ought to have a wider appeal than models. It is pretty cool see an EBT 2-8-2 or RGS #20 pulling a string of nicely detailed cars...working the injector, telling the fireman to shovel, and blowing the whistle.
I would like to build some models for TrainZ, but my time usually ends up being spent on books/magazines or working on my On3...too many South Park passenger cars to build in 1:48 scale to get too into v-scale.
Michael