My pre-order arrived yesterday and I set it up last night after the kids went to bed.
I had called the Apple Store near me yesterday afternoon and was told accessories would be available this morning when they opened at 7 am and that if you were not buying an iPhone, no line, just walk on in.
This morning I went by said store at 7:45 on my way to work and the lines for reservations and walk-ins were crazy.
When I walked up near the doorway I was greeted and informed I could not buy a bumper till they officially opened at 10 am. I explained that someone told me I could and all I wanted was a bumper. Eventually after showing them my activated iPhone 4, they allowed me to buy one.
As far as the bumper goes, it looks nice and gives good protection. I also ordered a bestskinsever.com back skin yesterday that hasn't arrived yet and skins the back glass and the side band. I am thinking I may use that and keep the bumper to use only sometimes.
As one of the reviews noted, the new iPhone feels solid, looks great and is Lexus-like, making the 3G/3GS look like a toy. The bumper removes a bit of that distinction because you can't see the metal band anymore. But on the flip side, it gives it a nice grip and lifts the back glass off the surface you set it down on.
Warning: Only the Apple sync cable will plug in through it, you cannot use any other car charger etc. Which is ok for me because I have so many of them and my car charger is a power plug with usb jack so I will plug in the Apple cable to that. The other word of warning is that only the Apple headphones will fit in the headphone jack, like the days of the original recessed headphone jack, because the cutout in the case isn't that large. So, my car audio cable won't snap all the way down. Good thing I think I have an original recessed iPhone headphone jack adapter at home somewhere I can use to circumvent this.