We run a retail clothing store, so we use a professional steamer all the time. We rarely use an iron on store stock because it takes too long to set up, and then to cool off before storing away. But I can give you a good comparison.
I am not sure the steamer is faster than an iron per garment - maybe a bit - once it is running. I takes a minute or so to start producing steam, and little time to shut down. It does a great job at removing wrinkles in many synthetic fabrics - the ones you use lower to medium temperatures on with an iron. It is quite poor at linen or cotton that requires high temps. Well, cotton T-shirts and stuff it's good, but it does not completely remove all the wrinkles, just most of them. On linen it never gets them all out, and even on some other fabrics. Sometimes going over the item slowly, to allow maximum heating and steam penetration, helps with tougher items. On any garment, you have to learn how to hold the piece stretched out flat so the steam can smooth it. I will not remove wrinkles in a section that is folded on itself - it may just enhance the wrinkle that way.
As someone else pointed out, there is no way to make or re-sharpen sharp creases with a steamer. And if you really want anything to look flat and pressed, you can't beat an iron. But if your garment needs a softer look almost wrinkle free, a steamer works well. And it is good for low-temperature synthetic fabrics - you cannot burn anything with a steamer.