In brief: been a BlackBerry user since 2006 and had a 9900 for a few months. The battery life is shocking. The device freezes every so often (and needs a couple of minutes to become usable). My carrier asked me to do an OS upgrade to see if that fixed anything, prior to them replacing under warranty.
In short, neither issue is improved with 7.1 (it possibly freezes less often, but still does).
However, I purchased an aftermarket 3500 battery (which is used with a decoder) and this battery appears not to work with the new OS. Could it be that security against unofficial batteries has been increased?
Is it possible to purchase an official high capacity battery?
To advise, with the hi-cap battery I will unplug it at 7:00am and it lasts until 17:00 - with the lo-cap it lasts to around 14:00.
I do most things to minimise battery use: I am on 2G only most of the time, I lock the screen with the top button, I close down unused apps, log out of apps that 'poll' ie facebook, and I use minimal GPS.
I do however have a bluetooth headset connected all the time (can't change this).
