I have jailbroken iphones/ipod touches/ipads since the early beginnings of jailbreaking. It has come a long way, that's for sure. Used to take DAYS to finalize a jailbreak. Now it's just the click of a button.
After all my experience, I can say one thing is for sure, it's a compromise. The biggest benefit, imho, is the ability to use Installous to install appstore apps for free. And that really is a great benefit.
The bad part is that it slows down the phone and makes it less stable (more prone to glitches, freezing, crashing, etc.). Every single jailbroken iThing I have handled, and trust me I have handled dozens, every one is slower and less stable than the non-jailbroken equivalent (unless you're a ******* and installed iOS 4 on your 3G iPhone). It's an inevitable side effect of jailbreaking. If you can deal with the instability and unreliability, then go for it!
After jailbreaking for 4 years, I thought I knew what Rooting an Android device would be like. To my surprise, rooting made my android device faster and more stable! It's like the anti-jailbreak lol...I don't think you can get free Market apps through Rooting though, although I could be wrong.