I tried the blackra1n jailbreak a couple of weeks ago. Wow amazingly easy and quick. I have since restored my phone to a pre-jailbroken state. To be perfectly honest I have not found a compelling reason to run jailbroken since 3.0. But I like to see whats happening on that front every once in a while.
I want to muck about in the sdk and I want to take my muck and move it to an iphone. What I am doing is not worthy of sale – or really even distribution, but to test it on an actual hardware I have to pay apple so my app is signed.
The simulator is exactly that, a simulator, not an emulator. You really have no idea of the performance of your code until you throw a real iphone in the mix. But with a jailbroken iphone and a bit of pokery you can get the sdk to play nice with your hardware. See this page: http://www.246tnt.com/iPhone/.
Well worth the effort. Now I know my app which runs fast and smooth on the simulator, waits 30 seconds and then crashes my iphone. I used to be able to do that on my Windows Mobile without coding anything.