In our trips to the US for NFL games we have bought tickets from both Stubhub and eBay. In each and every case we did spend more buying them early from those sites- but I did that just for the knowledge that our tickets were secure and we wouldn't be spending $600 on plane/hotel just to watch the game at a sports bar.
I agree that you should be able to get tickets closer to the date, and for a better price. Use all your avenues too- like be sure to have a WTB post in the ticket sales section here. (Following the guidelines of course.)

Because I know if I have tickets to sell I would rather sell to a fellow CPer that I know and know will enjoy the game than just some dude at my work.