Had the jailbreak with the iPod, but the iPhone is not jailbroken.. just don't feel the need for any jailbreak-only apps atm.
The biggest plus of a jailbreak was multitasking, well now we've got multitasking (or at least fast app-switching).
A jailbreak is nice if you want information like weather, calendar, etc on the lockscreen, though.
The jailbreak made the iPod really slow so I decided not do do it on the iPhone4.
I'm not shure if you get flash with the jailbreak, btw. I know there was a beta or an announcement but I never read of an app.
Truth is, you will virtually never come to a point where you miss flash.