Shortly after the arrival of iOS4 I got complaints from friends who'd updated their 3G phones only to find them running slower and slower. I had personal experience of this while using a friend's 3G device and vowed to tell owners of the third-generation phone to avoid the upgrade. Meanwhile, iOS4 runs extremely well on the more powerful 3GS model and is definitely a recommended upgrade.
Now, it seems, we can recommend iOS4.1, the latest version, to 3G phone owners. Lifehacker has posted a video comparing iPhone 3G speeds when running iOS 4.0 versus the new 4.1. iOS 4.1 won all the four tests, some by as much as 10 seconds. Some of the features of iOS 4, including multitasking, are disabled on the 3G. Personally, I find multi-tasking something of a disappointment, mainly because of reduced battery life. Since any application you open then remains in standby mode, any program making use of location services or alerts can seriously harm your battery life.
Also, I find that I have to housekeep the multi-tasking feature every day or so. Unless you reboot the phone, you can find dozens of applications still in memory when you double click the home button. To close them one by one you need to press and hold one running app until all the icons start to jiggle and you can hit the X button on each. Of course, this is not the same as removing the apps from the device, so you need to be sure you see the multi-task icon strip at the bottom of the screen before deleting.
So 3G phone owners are not missing much and I can now recommend upgrading. iOS 4.1 is due in the UK Apple Store on September 8.