[zumbojo]

The upgrade is practically automatic. iTunes nags you every time you plug in an out-of-date iPhone and only requires a single click. That and the jump from 3.3.1 to 4.x is an absolute night-and-day excruciating difference in speed and usability.

[rimantas]

It's not 4.x, it's 4.0. 4.1 and 4.2 beta run just fine on iPhone 3G.

[msbarnett]

Only the jump to 4.0 is excruciating; 4.1's performance is on-par with 3.1.3

[bryanh]

It merely helped in my situation. If 3.1.3 ran at 100%, 4.0 ran at 40-50% and 4.1 bumped it back up to 60-70% at best (in my experience).