Duplex really does save you paper - it's not just marketing hype. If you print regularly, whether it's school stuff, recipes, craft templates, or whatever, you'll notice the difference pretty quickly. Automatic duplex printing is especially convenient because you don't have to manually flip pages constantly. I probably feed 50-100 pages through my printer each month, and with duplex I'd definitely waste a lot less paper. Ink savings are less direct though - the amount depends on the number of pages printed, not how much paper you actually use.
The pitfall a lot of people overlook: these devices cost more upfront and repairs are pricey too. If the duplex mechanism gets stuck or acts up (which happens more often than you'd think), it gets really annoying and expensive. Some cheaper models have problems with this, so I'd go with established brands for automatic duplex, even if they cost more. So it really makes sense if you're actually printing regularly, multiple times a week. If you barely use it, it's just a fancy extra feature you don't need.