I really need very few apps to make my ipad WELL worth the investment.
Books/magazines/newspaper: I read a lot of books and like to browse magazines while on the treadmill, this satisfies this need quite nicely without having to lug around a single book or magazine. I get a LOT more also than I would at my newstand, things like NPR (fabulous program/content) for example.
Internet: I can get most of the functionality of the web on this device, sure it's missing flash and that does prove a detriment sometimes, but overall for the convenience of leaving my laptop at home it's doing a great job.
PIM: As a PIM it's very nice, having my datebook/calendar, contacts list, etc synced up and ready to go it quite nice. Arguably this can be done on most smartphones just as well sans the big screen, but I'm an old schooler and still remember the days when we carried around physical datebooks we wrote in.
Notetaking: This is really the single thing which stands out as a huge FAIL on Apple's part. This is the one achilles heel and the one thing I miss the most, is the ability to quickly, accurately and smoothly enter in gobs of text whether I'm at a seminar, class, work, or just creative writing. A bluetooth keyboard helps out a ton, but then you have just left the "ipad replaces everything" zone and it's not really in the same category anymore.
All in all I don't care about 99% of the iphone programs out there, 99% of them are pure and utter junk. I see the same trend happening with the ipad. But in judging the ipad (and iphone) on 1% of the apps it's a clear winner, and only in a small part to my list above (remember this is for ME). Once someone solves the text input issue it will all fall into place for me, but as is the ipad is well worth the investment for me.