To buck the trend...running T-mobile Froyo...only had it for 3 days, but have noticed a considerable improvements - was on the train from Liverpool to London, watched videos the entire time, with 25% screen brightness, internet on, auto sync...started on 48%, 2.5 hours later, had 21% - pre-Froyo my phone would never have handled that...
I know it's anecdotal, but seems a marked difference to me. Although I have found my battery life has been improving all the time anyway - it's true what they say about "wearing in" batteries!