I use Exact Target, every day, and I'm very happy with their product and their service. I won't speak to the valuation, but their technology and application is great.
You would call their technology and application great? I use it every day as well, and for me it's one of the worst pieces of enterprise software I have to use. Unusably slow, and what should be very simple makes me jump through multiple hoops.
I don't know if I would call it "great" it is pretty awesome. But it's also slow as you can possibly be. There are times it's unusable due to the slowness.
What's interesting is that the Salesforce.com integration is pretty poor when compared to other tools (including Pardot which they own). So Salesforce bought the tool with the worst integration. In contrast, Marketo has probably one of the deepest. So clearly more about revenue and customers than tech.
Using their application makes my internet connection feel like a dial up modem. Backend integration is horrible, API calls can take up to 5 minutes to get a response back.