Sorry, I'm skeptical of the the thread starters intentions, he/she joins recently and their first posts appear to be all adverts for Sling.
However to answer your question, your performance on wifi should be fine, your performance on 3G is dependent on how good your bandwith is, both on internet provider and ATT. If you use the recently released optimized app for the iPad, and you do not have good up/down speeds on your cable provider, you will be prone to freezes. I have the worst of both worlds, I mostly use Slingbox going to and from Midtown Manhattan (ATT), and I am on Cablevision.
There has been mention of this on the Slingbox forums, especially when the app first came out.
I currently have a 3 year old Slingbox AV and a Slingbox Pro HD, however my speeds suck, so I have both the iPhone and Ipad apps on my iPad, so I can switch if I find poor performance from the iPad version.
Of course I am not recommending buy both apps, I already had the iphone version from 2009.
Once again, mileage will vary from person to person depending on their internet provider. I just want to alert you it is not necessarily a slam dunk.