What's $3.1 billion between friends? Or, put differently, what's it worth to fix your display-advertising problem, corner the market for the "advertising operating system," and deliver a hammer-blow to an already prostrate Seattle-based competitor? $3.1 billion? Sure. Only a few quarters of free cash flow.
So now Google controls a vast share of the market for graphical online advertising, too. And has yet another display on its world-domination dashboard about who's doing what where. For those with an eye on the really-long term, it's hard to see how this isn't good news.
For those with an eye on the near-term stock price, meanwhile, it's probably bad news: Lower margins, a big management challenge, a significant price tag, an admission on the largest scale to date that it sometimes makes sense to buy instead of build (no shame in that--just lower returns on capital), and so on. But as Google made abundantly clear in its IPO prospectus, management (sensibly) isn't focused on the short term.