"Salesforce has done this extremely effectively, from salesforce automation, to customer relations management, to customer service, etc. In many cases now it is virtually unimaginable not to use Salesforce, since the feature set is so extensive."
This is the same strategy that Microsoft has pursued for years - and it sometimes works and sometimes doesn't - MSFT going after Intuit comes to mind, as does their own Salesforce.com competitor.
ExactTarget is a good large-scale email tool, but it is a very below average marketing automation tool. Now, they purchased Pardot, which is a decent, if basic marketing automation tool - good for small business but nowhere near an Eloqua or a Marketo - more comparable to Hubspot. So the question is how integrated they can make all these various tools. To date the Pardot/ExactTarget integration was rather weak - probably more to come on that front. I could see a world where they use Pardot's front end and ExactTarget just for scale email sending but the existing user base won't appreciate that move.
Now, I look at the integration (or lack thereof) with existing Salesforce acquisitions, and I have little confidence that Salesforce will be able to build a world-class, fully integrated product. For reference, look at the integration of Buddy Media or Radian6. So Salesforce may have all the tools, but true integration between the tools has been historically lacking. Now they're adding a company that is strong in B2C email and has a small B2B marketing automation tool that they themselves are trying to integrate. Sounds very complex and difficult - and sounds like something that would take a number of years to get right.
The real question for me is what now happens to Marketo and Hubspot - Hubspot, for example, has an investment from Salesforce, but was probably unavailable for acquisition as they want to spin public. Now that Marketo is public, and it looks like their business is not very sustainable (based on my read of the S-1), they'll likely see their stock price dip to the point where they look more interesting as an acquisition to someone. They're valued at around $800m right now - so if they show poor financial metrics, I'd expect that to dip. If they get cut in half, then a pick-up in the $600-800m range becomes viable - about the same price as Eloqua - but I'm not sure who would be the buyer at this point.
I think Salesforce did the right thing in terms of topline revenue (ExactTarget does around $300m vs. $65m for Marketo) and customers (ExactTarget has large B2C brands, Marketo mainly B2B), but they bought a far inferior product. Maybe they pickup another company in the space for the product?