I was first introduced to WhatsApp last year while I was in the UK. It's a pretty nice app IMO (and is apparently popular enough such that Mexican carriers offer an entire separate data bucket for it, so I'm not sure the FB stuff will have much of an impact in the short to medium term).

The only thing I'd change thus far is to make it easier to switch platforms; moving my messages from iOS to Android recently was a huge pain and ultimately required me to buy a third party app from the Play Store. Though to be fair, this is likely not a common occurrence.