i agree with iphoto being the most used one. although i would not say that mobile me should be free, at least the server space and excellent web based email system. what should be free though is the syncing of iphone/ipod touch contacts/calenders etc with your mac, considering what google are releasing it will most likely come to that eventually.
for the apps, well iPhoto is most used because everyone has pictures and cameras no (pretty obvious). but i think quite a few people that go on holiday and take a video camera will use iMovie. Ive only had mac computers since september (cant believe it took me so long to buy one!!! i think they are great) so ive only been able to edit a few movies. one actual big one about 30 mins of footage and it was very easy to pick and learn. not really that hard to do everything in the program.
for iDVD i can only presume that it will only be used if you use iMovie, since they link in well together. that is also easy to use and if people relised how simple it is to make a fairly decent home movie and then add the excellent effects of iDVD, more would use it.
GarageBand, well this has been fun to mess around with, although i have only really done magic garage band and not the real deal. mainly dont really have much time... but still very good easy program - not sure about the teach your self thing. planning to start guitar next year so i shall see then (from a proper teacher)
iWeb, well im part freelance web designer (part because i am studying at uni) and dont really see the point in using iWeb, so this is the least used app out of all of iLife. although if you have just had a special moment in your life (child being born, marriage, won the lottery - anything) its a very quick, efficient way of getting your message across that allows more flexibility than a blogging site. considering that you dont need to have a mobile me account either, ie you could actually host it on your own machine if you set up server access and point your domain name registrar to your machines IP.
all in all i think ilife is a very good set of apps, much better than what windows offers (essentially windows movie maker, windows media player (sort of) and front page (not sure if thats standard with the OS, dont really care either)
but of course you can get better apps based around things from iLife, and Adobe apps are probably some of the best (photoshop/lightroom (for editing better IMO, but not for storing and viewing), dreamweaver, sound booth and from other companies as well.)