Make sure you check the expiration date on the $40 off coupon. The gov allotted a set amount of $$ for those, and sent out coupons until they ran out of money. Now there's lots of people on a waiting list. If your coupon expires without being used, a coupon will be sent out to someone on the waiting list.
According to what I've heard, the digital signal is stronger and clearer than the old analog signal. You may get more local channels with the box, or the channels you get should be clearer. I've also heard that the digital signal will allow stations to broadcast more than one channel over the same signal, so your local station could, conceivably, broadcast sports or and other programming at the same time, over the same signal, to different channels.
(My NPR station has some local programs that I listen to while I'm at work. One of them has a monthly visit from the tv critic from the local paper, and he's talked about this switchover every single time since it was first announced. I'm glad the switch will happen soon, so they can stop announcing it!
I think Obama wanted to delay it, to give people more time to get their boxes, but I don't know if that will happen. Hawaii has already switched over to the digital signal, a month ahead of everyone else.)