The wall charger that it ships with is a USB to wall adapter. I charge my 360 either connected via USB to my laptop, or plugged into the wall with my iPhone USB charger. It works perfectly fine and I wouldn't spend the extra $20 for the included charger.
If you don't have an iPhone or any other USB to wall charger, you can probably pick one up for cheaper than $20 locally.
Having said that, I'm in Australia and the wall charger was not an option for my country. Had it been, I might very well have bought it, not knowing that the iPhone charger would have done the same thing