I want to second reading the Don't Panic! guide.
The advantages are many. You can make full backups of your entire phone in case a software glitch screws you up. Many apps have additional root options, such as alternate launchers allowing widget adding through the app drawer. And, of course, there's the customization and ROM options, which it sounds like is a big motivation for you. If you want Jelly Bean now, you need to root. Additionally, from a personal development standpoint, you really learn a lot about how to use android. Iv'e learned more about organizing files and transferring them to my PC, and how to copy them back to the phone myself, then I ever would have guessed. You learn how to structure the files so that they work correctly, how to copy media back to the correct folders on the phone, and how to utilize recoveries like clockworkmod to really control your phone.
To briefly sum up the cons, there's a few tools out here that make most of the potential downsides less awful or mitigate them all-together. If you use Wugfresh's toolkit, you can back up and restore just about everything. This means that, when you unlock the bootloader and lose everything, you can at least get most of it back with a minimal amount of setting things up. Additionally, it takes all of the real guesswork out of installing PC drivers, which were never working correctly for me, but now are installed and perfect. And, as the Don't Panic! guide cannot stress enough, if you happen to brick your phone while flashing that much desired Jelly Bean, there's almost always a solution to the problem. Especially if you're going through the steps in the toolkit and clockworkmod, you'll always have a just-in-case available to you. Attention to details is a must, but If you read through the stickied guides and take things one step at a time, it's all good.