Signature Edition, I think, is a brilliant idea. It's a great improvement on the standard Limited Edition model that video games have used for years at this point. Normally the really die-hard fans pay anything from $10 to $40 extra for their special packaging and extras, and in many cases a number of those special editions end up just collecting dust in the stores until their prices are lowered a number of months later.
With BioWare's model here, though, the special content and packaging comes absolutely free, there will (in theory) be no leftover boxes collecting dust in the stores, and all you had to do to guarantee yourself the whole package was to pre-order the game at some point in a 3 month window.
Then there's the regular pre-order DLC. Not as spectacular as the package you get with the Signature Edition, but they seem to be fancy (yet non-essential) bonuses. A leveling sword and a shield with an XP bonus.
And, finally, there's the "buy the game new" DLC. Besides the extra character that comes with Signature Edition, this is probably the most valuable of the extras: The Black Emporium. Basically a shop where your character can purchase the game's most powerful equipment pieces. Doesn't matter if you pre-order the game, buy it once the reviews are out, or wait a couple of months before picking it up, as long as your money actually goes to the people who created the game for you.
It may seem a bit much, but I think it makes sense. Fans who pre-order far ahead of time are rewarded, people are still given incentive to pre-order closer to the release, and people who buy the games used are penalized for not supporting the developers and the industry.
EDIT:
Funny thing for me, though? I haven't paid a dime for my pre-order yet. I could cancel it a couple of days before the release, if I wanted to. Heck, if I'd made the order and said that I wanted to pick the game up in their store rather than have it sent by mail, I could have just decided not to buy the game and still get the pre-order DLC codes sent to my e-mail account. I'm not one to abuse awesome retailers, though, so of course I'm going through with the purchase.