The app does work and it very handy when it comes to quickly communicating with your customers. Its really faster than using old web interface in certain circumstances.
But there are huge issues with both iPhone and iPad versions. For starters, iPhone app ticket view is just plain horrible. You type your comment on one screen, than you "save" it and than you push the submit button which actually submits the ticket. Again, loud and clear, theres no way to post an update to a ticket from one screen, you need two screens to just post a comment.
Secondly, the app just messes up with most Apple HIG terms. Itd be so much better if it didnt.
And as for iPad app — its much more handy than the iPhone one, but it lacks Retina support as of august 2012. Come on, guys..
xnutsive about Zendesk, v2.1.11