After 4 years, my battery finally started showing signs of weakness so I decided to replace it. I first called the local $tealer and they quoted me a price of $350 for a new battery installed. They said if I wanted to buy one over the counter and install it myself the price would be $200.

After checking out some of the local automotive stores in my area, I picked up an East Penn battery from Bennett Auto Supply with 900 CCA for $109 and installed it myself. The battery was a perfect fit. East Penn acquired Douglas' battery division back in 2005 and Douglas had been the supplier for many, if not all, of the bmw branded batteries in the US so I figured this would be a safe bet.

Install went fairly smooth and took about 40 minutes. I did not lose any of my memory settings (except for the time and date on the nav unit) which was a major concern after reading some of the posts here - no warning lights staying on, etc.

All in all, if you need to replace a battery and you are somewhat handy, opt to do it yourself and save between $100 - $150. It really is as easy as has been documented by others here.