I've only used the app for 5 minutes, and it's obvious how much thought they put into the design. The feature set and presentation are exactly right; when I say they left room for improvement, I mean they did a great job and left me wanting more.
And it's worth mentioning that now that it's on the iPhone, Highrise might be my new favorite task manager. It has email reminders, and since the to-dos live in the cloud they're available immediately across all my devices.
Despite a certain amount of Rework/Jason Fried fatigue lately, I still think 37signals are the best there is at what they do.