1: http://venturebeat.com/2014/01/14/where-nest-ranks-among-goo...
Not to mention the enormous number of innovations they'll likely be able to churn out. Hopefully it's like an AI focused PARC, but with a competent tech company at the helm :-)
Though if they're expected to do wonderful things, and they've been doing things for years... it seems a certainty that they have already done some of those wonderful things. And hence have something concrete worth acquiring. Which would explain the valuation.
There is a difference between Snapchat being worth $3 billion and Nest being worth $3 billion. The former gets the valuation based on users, the latter on talent and intellectual property.
Ditto here: $400 million is not buying you users, it's buying you raw talent and IP. Users can go off to another service in a blink of an eye - IP can't (talent can, but you can often structure the deal so that it won't for some time).
This could still be a terrible deal (I'm sure there are some people at Google still a little sore over Motorola, where the IP was valued far more than it ended up being worth), but for very different reasons.