We have a few spin bikes in our gym at work, and it was kinda of cool to be able to use them with Zwift this morning.
I have a BLE PowerCal which provided the Power and HR numbers to my iPhone 6 Plus running the Zwift iOS app.
The combination worked pretty well for a quick 40-minute ride at lunch on a rainy day. It certainly beats mindlessly staring at the wall.
Although the PowerCal is pretty suspect accuracy wise, it really is nice that it along with your iPhone will allow you to use Zwift anywhere you have a gym/spin bike.