I am a this app desktop user. I was hoping that Tableau would justify my company's investment in this app Server, but I am thus far unimpressed. As far as the UI is concerned, they have done a pretty good job. Two things, however, killed it for me.
1) They go to all the trouble of allowing me to select and view a set of data, but don't allow me to share it with anyone. An email feature, or an export to Dropbox, Numbers, Pages, etc would be excellent. I need to be able to collaborate.
2) Prohibitively slow. I (and many of the execs and sales reps in my company) are running iPad 1s (maybe the 2 is better?), and the visualizations all take a VERY long time to load, and an even longer time to filter, manipulate, etc. If I don't have the patience for the data to load, I know for a fact that other users in my company won't.
While I still think this has great potential, I'll be waiting to see improvements in the next iteration. I have faith that the this app team can deliver, but this is not it.