having upgraded to iOS4 on my iphone 3g it has become painfully obvious why apply disabled multitasking on this device. the iphone became close to unusable after i installed iOS4 w/ multitasking enabled. performance of the device was awful and apps were prone to crash or worse lock up for a long time, make the phone unresponsive, and then finally crash.

the phone crashed on me numerous times. i’ve finally had enough of it and restored it back to the virgin unaltered state and tried again without the hacks. the device is much more responsive, but still feels a little slower than in 3.1.2. all you really get with iOS4 on the 3G is a unified mailbox and folders. that might be enough of an upgrade to try it out, but man, battery life also feels like it’s not quite as good. it could just be due to me using the phone for a more prolonged period, but i don’t think so.

one thing of note, for those who are trying to jailbreak with redsn0w. if you install iOS4 and then use redsn0w for windows 0.9.5b5-4, you will lose MMS. the fix for this is to jailbreak your device like this:
1) clean install of iOS4 via iTunes.
2) jailbreak w/ redsn0w 0.9.5b5-3 w/ Cydia installed
3) jailbreak w/ redsn0w 0.9.5b5-4 w/o Cydia installed

that being said, i don’t know if i would really hurry to upgrade to iOS4 if you are a jailbreaker. tons of jailbreak apps don’t work and i’m not convinced it is worth the hassle yet.