Yes, you're getting a bumper for free, but why not wrap your iPhone 4 in something a little more stylish and original? Vaja has released a line of its great premium leather gadget cases for the iPhone 4 -- I love these things, as they're made out of high-quality premium leather, and add an extra little bit of class to a device that already feels like a luxury phone.
I do have to say that I don't really like that flap -- I use a Vaja case on my iPod, and in that case, the magnetically-fastened flap comes in handy, since I don't usually have my iPod open. But on the iPhone, I interact with the touchscreen so much that the flap would get in the way. But Vaja also has some excellent cases (like the iVolution Grip) that just slide onto the iPhone and stay out of your way.
They're not cheap -- the Grip runs $65, and they go up from there. But they are a quality product. While the bumper cases are all right, if you want something really nice to wrap your iPhone in, give Vaja's cases a look.