I did it on my son's iPod for him but it was an untethered jailbreak and he didn't like it so I restored it back. Swore I'd never bother jailbreaking again.
Surprisingly I found Vlingo so will see how it goes. Thanks for the tip.
Just ease of use. I like the Siri function on the 4S but have still have a while to go on my iPhone 4 contract so not upgrading for a while. Will probably end up getting the 5 by then anyway.