Just now i was chatting with my college friend in Marathi - one of many languages spoken in India. I had missed some of last sentences so so opened it again from "Chats" section in Gmail. Google shows clips and sponesred adds above the main area where mail is displayed. I was amazed to see Adds of site targeted to marathi speking community, to be specific : Matrimony site. How does google know that i am chatting in Marathi and Engilsh mixed and Matrimony site add is relevant to the item to be displaced. Hats of to google!
But at the same time i feel you should be careful about GMail and Google in specific. They are building database of people : your social network, your browsing habits and many other things. Now it has come-up with browser - Chrome. Minimalist one, but behind it Google can store lot of personal information on its servers. Google's mantra is "Don't Be Evil". But after certain limit its really intrusive. Google basically uses all this information for advertising thats there bread-n-butter. But who knows what will they sell to the advertisers. So Beware of Google!
Here is post on HackerNews Network that talks about Dependence on Google