From the point of view of advertisers, this announcement is definitely A Good Thing. There are clear benefits and there seems little doubt that Google needs the competition. So in the short term we can all celebrate.
In the medium term the question is whether the new partnership will build or lose market share. Google has an enormously powerful brand that people have learned to trust. I think it is fair to say that “Bing” is not currently in the same league. Bing has made a good start since it launched, but it’s been supported by one of the biggest online marketing campaigns ever. Will users switch from Google? I don’t know. Will you?