Well, spinach is boring, so your friends are at least right on that account.
What you really should be doing is enjoying all of the delicious greens that are coming into season now. Arugula is fashionable (and delicious), but don’t overdo it. The best way to enjoy greens is to eat what’s freshest and locally available at any time of year, and to mix it up. (If you find arugula boring, you are probably eating something less than in season.)
Greens are good, and nutrient packed, and will generally make you live for like 500 years if you eat them, but it’s easy to burn out on them. Your friends’ contempt for spinach has more to do with it’s ubiquity than anything. Everyone knows that greens are good for you, but the selections available in most grocery stores and restaurants are limited, and what they do have tends to be extremely bland.
Farmers markets and CSAs are good sources the best and freshest greens. Try an organic farmer’s spring mix (in the spring) with a simple vinaigrette and bask the glory of the season, trying each different green in turn. Arugula is great, but so is mizuna and young red chard. In the winter, just accept that you’re going to be without the leafy delicious, and get excited when it comes back in the spring
Anyone who says that arugula is the best green is really saying “I don’t eat enough greens.” Eat healthy, mix it up, and keep it fresh!