Two iPhone "killer apps" and why I didn't upgrade to 3G
I'm an iPhone advocate and early adopter, got it on the first day it was sold last year, and couldn't be happier with the product.  (OK, sure I could.  The software can be "buggy," as Chip points out.)  I believe this product completely changes wireless, largely because it completely redefines the phone's user interface (UI) and delivers two killer apps to the phone.  

Killer apps for wireless are:
1) BlackBerry email.  However, they developed this experience many years back.
2) iPhone (version 1) wireless Web interace.  95% of iPhone users surf the Web regularly on their phone ... because it is a good/great experience.
3) iPhone 3G App Store, immediately loaded with 500 new apps, some useful, some not.

For this reason, I am completely happy with my non-3G version of the iPhone.  The App Store is fantastic and is the 2nd "killer app" that the iPhone has delivered, but you don't need to get a second version of the iPhone to get that service; it is available in the software upgrade.  If you don't believe me as to how cool this App Store is, download the Pandora app to your iPhone, create your own radio station, and start hearing your favorite music as you walk down the street  or work out.  The non-3G version also received access to Microsoft Exchange and Active Sync, so it is more enterprise ready, meaning your IT department can no longer use that as an excuse to shut you out from the.

There are advantages to upgrading to 3G, most prominently 3G speeds (very important, though a battery drain), GPS (though I love pseudo-GPS Google Locator on my iPhone), and improved call and spearkerphone quality and volume (wish I had that).  The biggest disadvantage of upgrading is that you have to pay more on a monthly basis - $10 more for data usage and hidden $5 more for SMS, total $15 extra per month.  At least for the moment, I'm happy with my OLD iPhone.