I love imessage because it basically allows me to message anyone from any of my apple devices (I have iphone, ipad, macbook air, imac). For instance, I can have a friend message me on my phone, and I can reply from my PC (faster to compose a lengthier reply), or if I am in another room, I don't have to go and reach for my phone, but instead just reply from my ipad.
Also great for quickly transferring larger files (gmail allows only ~20+mb attachments, imessage allows up to 100mb).
Whatapp is actually a bit irritating, because I have to switch between it and my normal message app (I still have a few friends on phones which do not support whatsapp, or simply haven't gotten round to downloading it), and I can't do it from my ipad or macs.
Is there a reason why whatsapp isn't yet available on tablets and PCs?