"Well" depends on WotC's definition of it. As someone who doesn't really believe the "it's not 4.5" boilerplate, I think that what they're hoping is that it will do 4E core books sales + additional sales from new players and lapsed players. If it doesn't anywhere close to that area, we'll probably see 4.0 quietly forgotten in the product catalog and most new products being Essentials supplements.
As a starter edition of the game, it looks pretty good. I know a group of people from work that never gamed before and jumped in with 4E. It should do well in that niche, I think. If it does good sales at places like Barnes and Noble and Toys R Us, it could stay as an evergreen book in that category.
Unfortunately, I think that Essentials is an attempt at a a stealth 4.5. It allows WotC to have their cake and eat it too. If it succeeds like a 4.5, then they just keep pumping out Essentials stuff. If it doesn't, it's completely deniable.
My opinion: it won't sell like a 4.5, and will probably sit in places like Barnes and Noble or Toys R Us until stock of the first printing runs out, and that'll be it. It'll get killed if it doesn't sell like a new edition, regardless of whether it works as a starter product, due to it not living up to internal expectations.
Three previews were sufficient to make me want to buy Essentials.
The fourth preview made me want to make our next game Essentials ONLY.
You know, it's funny how much people's preferences differ. (Seriously! I'm not trying to make a dig at you!) I've felt the previews were, in general, inadequate to get a good feel for what the classes were like due to not knowing what all those class abilities were. A first three levels type preview is much better. But from what I have seen, every preview has made me like it less as an actual addition to 4E. After the rogue preview I officially banned it from my upcoming Dark Sun campaign.