I've been using Zwift a little with a smart trainer (Tacx Vortex) . it is quite fun but surprisingly hard, much harder than cycling outside. this is where it get complex because you are mostly cycling against a power, so speed is irreverent in zwift. When watching the screen it is very to get a feel for the incline which just gives you this feeling of putting in a huge amount if effort to not go very far. And the graphics on my macbook are quite basic so there is little sense of speed.
i also find it a little worrying that the effort levels seem to vary each time I set my bike up. Sometimes I can't average 120W FTP, other times I'll average 190 at the same effort. I think small changes to wheel alignment and tire pressure make a surprising difference to what happens. this kind of makes the whole competitive thing a bit weird, not least it is easy to cheat anyway by understating your weight.
you also get friggin hot and sweaty in no time. A big fan is a must, still deciding if I want a fan or a full blown portable AC unit to get ice cold air. My house has AC and I set it down a little but even then I need a bucket under me to catch the sweat!