From what I've seen/read (unfortunately not first hand) is that it's very good, quick and extremely well integrated.
I must say this though (and please don't take this is a personal pop) there's a lot of reviews on the iTunes store for WhatsApp saying "when the new iMessage comes out it will replace WhatsApp". They all seem to forget that iMessage is great if you and everyone in your contact list has an iOS 5 device etc, but WhatsApp is designed as a cross platform (iOS/Android/BlackBerry) app so can't fairly be compared to iMessage (although their core function is the same).
My thoughts exactly - I'll probably have to continue using Whatsapp as not all of my contacts own an iPhone.