Thought I'd chime in too. The wife's '03 330xi needed a jump this morning in 10 deg weather. So with the car being about 4.5 yrs old already, I went out and bought a Duralast Grp 49 battery at Autozone just to be on the safe side (she's already traumatized from a 2-flat tire day last year, the poor thing!).
Fits like a glove (almost better than OEM it looks like), better CCA rating than OEM, and easy on the wallet ($90). Good deal all around!
PS- I never realized how long the old battery took to crank before I installed the new one. Seems practically instantaneous now.