In my opinion the E-Jet is a very safe bet. As Dehavillanddriver posted, it can and now will be used where the 737 can't make money. If oil prices remain high I can see more 737's going back to their leasing companies over the next couple of years and being replaced by the E-Jets.
I liken it to the Qantas 767/737 combo...ie 767's used on major trunk routes whereas 737 used on smaller and off-peak routes, and since Virgin has a smaller domestic share than Qantas I can see the E-jets supplementing the 737. I know this is a simplistic point of view.
If things improve soon, then we'll see the E-Jet do what it was intended to do....open up new routes otherwise we'll start to see them do say Syd-Mel during the middle of the day etc.