Much to Apple's chagrin, jailbreaking your iPhoneisn't illegal anymore. What's more, Apple's maintained that doing so will lessen the handset's functionality. Will it act a little different? Yes. Will it do less? Absolutely not.
Jailbreaking your iPhone does two important things: it frees you up to use free apps not supported by Apple, and it lets you use the phone in ways that Apple doesn't want you to, or didn't think of first. Of course, the flip-side here is that your iPhone is, in effect, no longer an Apple product after you do this. You won't get any support from the company is something goes wrong.
That said, the early benefits of the iPhone 4's jailbreaking are already becoming apparent. Using the My3G app from Cydia, a "black-market" app source, users have been able to make FaceTime calls using 3G. Right now, the iPhone 4 can only do that over Wi-Fi. The visual quality itself doesn't look like it gets any better, but, if you enjoy a solid 3G connection, you should enjoy less stuttering.
It's a neat taste of things to come, but in the end jailbreaking your phone really comes down to how curious you are to stretch its features. The official App Store and the jailbroken alternatives tend to mirror one another — though the latter is usually where the quirkier ideas land first.