I'm probably unique - I went from the oldest, most outdated cell phone ever to one of these! LONG story, but... (don't worry, will be mostly lurking, not asking stupid noob questions...)
To answer the question of why I ordered online, the local stores told me sometime in May, and between the free car dock and $50 online rebate for switching carriers, it was enough to win the day for me.
Annoyingly, AT&T shut off my old phone yesterday afternoon, and when I called T-Mobile customer service they told me yesterday they "couldn't tell" if the phone was being shipped out or not. Luckily, I got notification today that it shipped landing Friday, but when I checked it says it is about five miles away and will probably be delivered tomorrow.
If you switched to T-Mobile, make sure to get the rebate!