Hi Gayley
Some people claim to use nettles as a substitute for spinach - could your visitors have been pulling your leg?
In any case, I don't think anything can substitute for the real thing - I love it, along with parsnips, my two most favourite veggies.
It's actually very good for you, with plenty of iron and good antioxidant properties to help guard against cancer. The only downside is that its oxalic acid content means that anyone with kidney problems shouldn't eat too much of it.
See
Botanical Online for full details. And remember never to cook it in water, which ruins its wonderful flavour - just shake the washed leaves half-dry and then simmer in butter. Mmmmm....!

Alec