I thought I would send out a short warning now that enough broad tests have run:
If you use your 2nd gen iDevice for more than productivity (means you use it for multimedia or gaming), do NOT, I repeat, NOT upgrade to iOS4!
You will regret it serverly because it will run significantly slower (startups take easily twice as long) and multimedia apps stand little to no chance to not crash in many cases because under the new OS, although all the nice features were cut for 2nd gen devices, the amount of available memory is even lower than on iOS3 already. Back then you at least had 25-30mb on the iphone 3g, now be happy if it gets above 20 ...
So again: if you like gaming or using multimedia apps, skip iOS4 for the time being (4.1 at least assuming apple has any interest to solve the problems instead of just making you want to upgrade even more desperately after trashing your device technically).
EDIT: I realized I have forgotten an important part and that is that the device is not only slower but the cpu etc is also more used so your battery life will suffer (for me lifetime tends to be reduced to 50% and less)
Marc ‚Dreamora‘ Schaerer