I just upgraded my early 2011 MacBook Pro to OSX Yosamite. That too without taking a back up. It took less than 2 hours to do the complete upgrade – downloading 5+ GB OS and completing the installation. The UI looks a lot flat – like Apple did with iOS 7.0. I can’t say if I fell in love the new look and feel. It feels a little bit faster than the previous Maverick version.
If I owned an iPhone, as the story goes, Yosemite would’ve been more useful for me. I can’t see what went beneath the hood, the look and feel update looks marginal. As I play around more with the features, I might discover some usability improvements. Or maybe not.