iPhone users debate amongst each other constantly about this. I took the plunge with my iPhone 4 after the warranty had expired.
Positives: Jailbreaking your phone does exactly that. Frees it. You can theme your iPhone in anyway you choose. You can also get most apps for free, and the best reason; you can download an app call 3g Unrestrictor which tricks apps on your phone into thinking there’s wifi while you’re on 3g. This means you can use any app while on 3g including facetime which currently only works while on wifi. You can also use your phone as a hotspot without adding it to your current plan. (shhh… you shouldn’t do that. If your carrier finds out they will order you to cease and desist.)
Negatives: It voids your warranty. Sure you can theme it, but you like the iPhone and the way it looks. If you wanted it to look like a Droid, you would have bought a Droid. Yes you can facetime anytime you want. However, you don’t want to. They invented texting so that you don’t have to actually talk to people when you don’t want to, let alone see them. Hot Spot? You don’t want to be threatened by your cell carrier.
My Take: I’m enjoying my Jailbroken iPhone 4 and I’m glad I did it. However, I’ll buy the iPhone 5 when it’s released, and I will not jailbreak it. Well, not until the warranty’s expired.