Personally, I use Adress Book, Dvd Player, garageband, iCal, iPhoto, iTunes, iChat, Mail, safari, and quicktime to their full extent. I have yet for a real use of iMovie or iDvd.
Minus iMovie, since I have FCP, and iDVD since I don't have a DVD burner I use every single application Apple put on my PowerBook. Yes, even Stickies, Sherlock and TextEdit.
I make extensive use of all the iApps, including Stickies, TextEdit, and even Sherlock (maybe not technically iApps but they come with the computer). I've found that you really don't need to go to 3rd party apps often.
I learned my lesson about big projects and iMovie, so things with a ton of cuts get done in FCE now but I still use iMovie a lot.
I use iPhoto (a lot), iCal (a little, but important when I do!), iTunes (runs my home stereo system) and my kids use GarageBand (occasionally). I don't have much use for iMovie or iDVD. I don't think of Addressbook, Mail, and Safari as iApps, really -- they're more like a part of the OS itself. (fwiw, I use all three, all the time).
I'm very iLife oriented but one application I never use is iMovie (and Garageband because I use Soundtrack) because of another trusty Apple ap, FCE. The iApp I use the most has to be iTunes. I'm obbssesed with getting artwork for all 130000+ songs I have, so getting any real work done is a challenge. I've tried using FetchArt but the darn thing didn't find even the most popular album covers on my HD.
I use them all. I use Safari, Adress book, Email (though I may switch to Thunderbird if I can), iTunes, iChat (Rendezvous only though), iCal (I absolutely LOVE iCal), I haven't used iMovie and iPhoto too much as I am waiting for my Camcorder and I do not have a Digital Camera yet.