I upgraded about 4 days ago and on my 3gs, not saying yours won't be different, has been amazing. When my phone hasn't been rebooted for awhile it would normally take 10 seconds for my messages app to open. Since upgrading it takes a normal amount of time. Seemed to actually speed up my phone a fair amount. To me it was an extremely worthwhile upgrade.
My upgrade date is October 13th, so I can't get the 5 until then, so it's nice to be back to a phone I'm not tempted to throw through a window.