As others have mentioned if you are just looking for a phone do not bother- the phone is actually in my opinion the weakest feature of the iPhone.
Since getting my iPhone when I travel I no longer take my laptop with me.
On my last flight my eTicket was scanned right off my iPhone screen- I checked in via email on the iphone and avoided the kiosks and line entirely.
Here is a rundown not of all the features but the ones I use most.
The Cons:
Price
Camera is mediocre (in 1st and 2nd generation iPhone- I do not know if the new one is an improvement or not as I do not have a G2S)
The Pros:
Navagation- It is a full GPS with built in search. (Handy if you are in a strange city and want to have a cup of Starbucks)
Email on the go.
Internet anywhere you can get a phone signal
Voice Recorder which can also email your recordings.
Good Video Camera (G2S only)
Can hold your entire music library
Watch movies and videos and podcasts.
Applications for everything. Here are a few I have:
App Sniper- tells me when an app goes on sale. There is a great web site that is even better but I lost the URL- so if anyone knows it please post.
Fluther
Tweetie- a twitter client
Audiobooks- I can listen while working out.
iRenMovie- runs my Netflix account easily
Classics- A bookshelf of classic books that bookmarks where you are at.
FML- always good to know that no matter what day you have had someone has had it worse.
Texas Holdem
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If you would use any of the above features I say the iPhone is well worth the money. When I washed my iPhone (it was tragic and terminal) I was forced to use a trac-phone. It only lasted three days before I cashed in the penny jar, dug through the couches for money and raided my savings to replace it.