I used WhatsApp for a while but really hated it! The only "positive" thing about WhatsApp is the huge userbase... At least on my Arrive (WP7.5) the app sucks... no real service in terms of "user wishes" and so on...
So thatswhy I am trying Hike right now. And I have to admit: I really love it! Of course there are still some functions missing and only a few of my friends use it, but I am sure it will change soon.
Hike is very fast (a lot faster than WhatsApp), all the messages arrive (WhatsApp used to forget some of them...) and so on...The best thing about Hike: it checks your contacts on the phone and compares them to the hike-servers. If there is a match, you know that particular contact is using Hike as well... so you can chat with them without paying anything... So basicly you don't need a new profile and new "friends" and so on... just download hike, register your number und your contacts-list will be updated constantly! (btw: whatsapp does the same...thatswhy almost everybody uses it...). Hike will need some time for all the others to start using it...untill then you have to wait and chat with the ones already using it...but I'm sure it will be ok soon!
So maybe give it a try..
Greetz