After Jb do not go crazy using themes. They DO slow down the iPhone.
I Jb for all that was mentioned but also for an app that everyone did not mention.
XGPS! Uses google maps and the app TURNS when your car turns on roads.
And by the end of the month they will have VOICE guided directions. The app as is works really good. Still not as complete like tomtom but way better than just regular google maps. Once voice direction comes out it will be great. Btw, I been using XGPS with BACKGROUNDER so when a call comes in the app won't shut down, awsome!
QUICKGOLD is another great app. Press the home button once and start typing WHATEVER you want and it will find it. Finds apps, contacts and more. And if your on the desktop say at the 8th page and you want to go to the 1st page, just tap the home button twice and that's it. It also give you a few recent used apps to launch as well. Nice app!
CYCORDER works very well and does not freeze and it's not slow as some guy here mentioned. I never have problems with it.
To ME, if you use THEMES you will run into problems and slow downs. Dont use it, just use the apps mentioned in this thread and you WILL be fine.
CLIPPPY (something like that) is another very VERY good app. It's a copy and paste app that goes into to your keyboard. Double click a word and slide your finger at what you want to copy. Than click on the 123 button on the bottom left of the keyboard. You will see a top row on top of the numbers which have, COPY PASTE STACK and CLOSE. Very cool app.
I can go on but it's really a matter of looking for the right apps that work and that you will actually use. To jailbreak I suggest you do a complete restore, followed RIGHT AWAY with the JB. Than play around with Cydia apps BEFORE doing anything else. Once you have found what you like and will use, RESTORE AGAIN! than download your picked apps from cydia!
I have use this method on every phone I have owned. Helps to get rid of many left over files....... that do not get removed. Do it this way and see how nice JB really is. Just PLEASE don't use themes, or at least use ones that are light on MB.