I've played around with their mobile platform and found that while you're on a Wi-fi connection it's fantastic... I felt uncomfortable about using it on 3G though, I don't think the convenience is worth the risk of losing connection and screwing up a trade. I don't use it religiously but there have been a few fringe scenarios in which it's been clutch.
I think iRunner might work on the iPad too.... I'd sure be interested in trying that out!! Christmas is almost here, right?
BTW I have an iPhone 4... I'd be willing to bet the further back in phone versions you go the worse the software is going to get, so that's something to consider.