Because Safari gets to JIT-compile JavaScript and UIWebView embedded into apps can't, this actually feels slower than the website (~0.5 seconds between clicking on a message and seeing it load? really?)
I really think I'll move back to using their Exchange-Sync for push together with the standard web shortcut for mail management as this is faster and supports multiple accounts.
(edit) On a related note: Doing these "let's add a native toolbar to my website and publish this in the AppStore"-apps feels nearly as stupid as when everybody felt the need to add flash intros to their website. There's next to no additional value (especially with the currently broken push) provided by such a half-assed pseudo-native app.
This does nothing but hurt both native development (by leaving a bad impression on what's possible with native apps) and the open web (by making people believe that a website is not good enough)