In general, I posted my experience for BBM
here. For me it is not worth it, but I might be missing something
Small correction - it also works on tablets with no 3G. I installed it on my Nexus 7 (wi-fi only) and used it. The draw-back of this is that you can not have WhapsApp active on both your phone and tablet linked to the same sim card. I e-mailed the people at WhapsApp and they might re-introduce the feature in the future where you can link more than one device to the same sim card, but no promises.
Can you not copy the install file from your handset to the iPad and install it?
You don't need a sim slot to get WhatsApp to work on a wi-fi only device. Done this less than two weeks ago on the Nexus 7 (wi-fi only)