The moment I used Keynote, I never stopped using it. I keep getting better and every presentation I have given has kept my audience glued to the screen. Each time I have to present, I make the people who use Powerpoint look lazy thanks to Keynote's elegant themes and transitions. This is one app I will keep using as it keeps getting better and better.
Pages for me has always been easier to use mostly because it is much more friendly when handling graphics in the documents.
I have used excel for years and yet while Numbers may not have everything it has (or appear to...), it is definitely a more pleasant experience. I think Numbers speaks for itself in usability when my 12 year old sibling was able to graph some data into a pie chart quite easily with Numbers and make the graph look terrific, while on Excel it led to confusion.
It's quite obvious that MS Office is the dominant one here, but iWork is definitely a force to be reckoned with. MS Office 11 for the Mac comes with Outlook which is joke compared to Mail and iCal that already come with every Mac.
There's a lot more options out there than just these like Google docs, so what do you think? I'll take the iWork suite any day.