I have a 3G and 3GS and the 4.0 update is great! I must say that it performs much better on the 3GS, even with the multitasking aside.
If you are having performance issues I suggest a full wipe and restore with 4.0 and then sync to get your data, apps, and music back, don't do a restore from backup tho or you will be back to square one.
Posted via RS Mobile