Just wanna share my exprience on iOS 4.
-Im using i3G, jailbreak-ed.
PRO
-enabled multitasking and homescreen wallpaper = dead slow
-enabled multitasking only = just nice. no more lagging.
-improved battery life. Standby mode with normal sms and call (30 sms and 10 minutes call) can stand up to 1 and half days. Before this on 3.1.2, not even a day.
-improved app loading time.
CONS
-eat up precious ram as most all the factory apps (Clock, Calculator, Settings, etc) will stay in the background. Have to manually terminate (thats why by default i3G isnt eligible for multitasking function).
To summarize, I'm happy with my iOS 4.