So you're just asking a blanket question, without any details? Do you currently have a smart phone? Do you currently have an iphone? What do you want your phone to do?
Frankly, very few people need an iphone. I have a buddy who has one, and all he does is talk on it and text. A dumb phone can do that....
Now the question is, if you have another smart phone other than an iphone, then YES it is worth the upgrade.
If you have a 3GS or older iphone, then YES it is worth the upgrade.
If you have an iphone 4, then probably not unless SIRI is really important to you.