As mentioned, there is a demo of SmartDay for download at the developer's website.
I find the concept of SmartDay very interesting, because most other to-do/GTD apps just list tasks, so you lose oversight quite easily when you try to plan your day or your week. With SmartDay, your tasks get displayed in the calendar view, which is especially useful if you have tasks that take a bit longer to complete. You can rearrange the tasks according to your other events – this is actually great. If you missed a due date/time, SmartDay just pushes the task further and reschedules your work.
This is a real killer feature. And well, this was the very reason for me to have a look at SmartDay. Exactly what I was looking for.
Unfortunately, when dragging and dropping tasks to the calendar, rescheduling, etc., SmartDay often felt unresponsive, clunky. Also the iCal integration didn't work, but maybe this is because it's a demo? The design isn't beautiful, it has its rough edges, both from usability as well as the look and feel, some things are a bit "glitchy" (graphics errors). Definitely not as polished as the developer's website.
But what I really don't like about SmartDay, and really makes me avoiding it: it isn't a standard Mac-like app. I don't like having to search for little icons somewhere in the app's window, I want them up there in a (customisable) toolbar, and I like an app where I can access Preferences, etc. from the menubar. Also I like to drag an app's window around, resize it, etc. I mean we're on the Mac platform, where you don't have to re-invent the UI, because OS X has actually a set of great UI principles. SmartDay's UI really drove me nuts, and I couldn't get used to it.
Apart from the UI, planing projects is pretty basic, no sub-sub tasks, etc., so unfortunately, this app isn't for me. However, it has great potential and a superb idea behind it – I will definitively come back and have a look how it develops over time.
Anyway, I really recommend anybody interested in productivity to have a look at the SmartDay demo (or the free online version) and evaluate for themselves. Task management is something very individual, and chances are, that SmartDay just fits you and your needs perfectly.