I'm currently running 3.1.2 jailbroken and was thinking about upgrading to 4.0 and jailbreaking but i've heard lots of reports of how bad 4.0 performs on the 3G. Should I just stay at 3.1.2 indefinately?
With a few tweaks(iMemory Enhancer, Backgrounder, Recents closer) You can get a lot of the performance back. Downside: Not all JB apps work w/4.0! I like using "Folders" and the app switcher. The last two apps(i.m.h.o.) are easier to use the the "JB" versions that I used on 3.13(Categories/ Proswitcher.) The "double tap" to bring up open programs of course makes switching better/ faster but also quickly shows which apps are "open" or "running in the background" Press and hold the icon to close is a nice new feature.
I'm going to be one of the detractors here and say that I'm quite happy with the performance of 4.0 on my 3G. I jailbroke mainly for sbsettings and mywi; both which work quite well on the 3G. Tonight, I enabled multitasking and must say that it works very well; as long as you keep the number of apps running in the background to a minimum (I would say 4 - 5 at most). Under 4.0, I couldn't get any of the app switching applications to work with backgrounder (actually, I'm not sure that they have all been upgraded to 4.0 specs yet, thus the probable reason why they didn't work for me). I find Apple's method of app switching and multitasking, as well as their implementation of folders, runs very well.
I have to admit, however, that I had to reboot my 3G a few times before 4.0 would run smoothly.