Whatsaapp (and GTalk) has completely replaced SMS for me. The only sore point was that it was not ‘Android’y enough – looked too much like an iOS port, but it worked well enough (and had enough people in it) that my forays into Hike were never long enough. The latest update fixes the design to be a lot more flat and pleasing to the eyes, and I’m quite happy to stick to it. (Plus I just ran into their post about why Advertising sucks, so doubly happy to support them :) )