My phone usage : ~2h listening music (all around 192kbps VBR 75% volume) with web browsing (edge, 3G) with enabled ICQ account.
~10min calls , 5 to 10 sms, then around 6hours on wifi , checking e-mails , downloading images ,working in terminal ower ssh (replacement for netbook ) .., unplugged from charger at 8 mourning , went back to home at 22 o'clock with 6% of battery

I thik this is not bad - but for sure I purchase Energizer XP4000 .
It fit exactly in my back pocket and with modified cable from telephone (twisted soft cable from phone to earphone) i can use phone without limitation (little worse handling, yes)
Price is same as two original BL-5J batteryes.
It's abble to make two full charges from empty battery (I think charging non empty battery at time of hard usage will extend this because of charging effectivity)