These are a few reasons I think you should consider if you're thinking about jailbreaking your iPhone.
It's Easier Now Than Ever Before!
Back when the iPhone first came out the jail break process was pretty tricky and a lot of people messed up or "bricked" their phones. One real nice thing about the iPhone is that it is easy to unbrick (unless you have tried to unlock it in the early days) you just put it into DFU mode and restore! That is it; you now have a fresh iPhone. There are several tools that can jailbreak your phone and some will unlock the old version of the phone. As of this writing the 3G phone can not be unlocked.
I recommend Pwnage tool for Mac and Winpwn for windows users. Yuo can find more info about them at: http://blog.iphone-dev.org/
This is a fairly easy process and several sites including http://sleepers.net/news/ and modmyi.com have tutorials if you need them.
Quickgold
I have only been using Quicksilver on my Mac for a few weeks now but it is really an amazing little application that I can't live without anymore. Quickgold is the iPhone version of this program, but from different makers. You invoke it by hitting the home button and a search screen pops up. You start to type the name of an application, person or even start typing in a phone number. The newest version even allows you to google or launch a web page. It searches just about everything on your phone including bookmarks. All you have to do then is click on one to launch it. I am not sure but I think this version even learns what you go to often.
My only complaint is that 1) I wish you could find email addresses to send off an email and 2) a preferences; I don't like to search my bookmarks.
All in all a great and useful little app especially the way my iPhone is set up.
Categories
One of the reasons I like Guickgold so much is that I have a lot of apps on my phone but also I use a program called categories. This program puts folders on your iPhones desktop so that you can store and organize all of your apps. I have five folders and a ton of apps. So instead of having to swipe across many pages to find the app I want I can go into the folder I put it in, for example Games. (Or I just use Guickgold.)
This program really helps me keep my home screen organized and looking good. I like the simple life and uncluttered. The only things I keep outside of categories is the items I have on my dock (phone, email, calendar, and ipod) and things for iPhone.
WinterBoard
WinterBoard is a wonderful application that allows you to change your background and your icons look. It also allows you to download and change other things like the phone pad, your notes page and many many other things as well.
Cycorder
A lot of people have complained that the iPhone can't create video. Well it can now with a really good free application called Cycorder. It uses your camera and makes really good quality videos.
Intelliscreen
I used to use a Palm Treo before I got my iPhone and the one thing I really missed was the "today" screen. Luckily, Intelliscreen takes care of that gap and brings this function back to your phone
These are just a few of the reasons I jail break my iPhone to really customize and use it to its full power. all of these applications are available through Cydia. Cydia is like the app store for jailbroken phones.