the acquisition is worth it for two reasons: 1, google has the money literally sitting in banks, they aren't stretching to make the purchase. 2, google is notorious for failing to build cool shit that ALSO builds a userbase...groupon is nothing special, they could easily engineer their own. They're buying the brand to secure a solid spot in the local advertising space.
Groupon is a massive email list. Google is probably drooling over it. Look at Youtube, they left the branding intact, most people don't even realize it's owned by google now.