Just replaced my 5-year old factory 90Ah battery with a $99 Walmart H8 - an exact fit, alomost the exact weight too. Many posts suggested 49R is the correct fit, even Walmart's little electronic guide tool says so, but in reality 49R is shorter than the 90Ah factory battery.
The battery compartment in my E90 has multiple mounting holes so obviously the smaller and more expensive 49R would fit too, but why spend more for less?
H8 fits many European cars so it has two vent holes, one on each end. The unused one should be plugged.
The 4x4 and brake warning light came on after I started the car, but turned off before I reached the end of my driveway.
The charging system appear to work normally - 14.9V right after starting the car, then drop to 14.6. It occasionally drops to 13.8V at idle as if to check the battery's open circuit voltage (I imagine). I have monitored the car's charging behavior on longer trips and can attest that there are indeed some intellegence in it. The voltage drops to less than 14V on longer trips, but this is not unique to BMW.
In the name of science, I will not register the battery. I'll report back in one year.
I think the whole registration thing is overblown. Flooded cell lead-acid battery was invented before the Civil War and has been in wide use for more than 100 years. It's behavior is VERY well-understood.
Under some circumstances registration is needed - your car has the pseudo-hybrid "efficient dynamics" or has automatic engine start/stop - the car does need to know exactly how much charge the battery has and can hold.
The registration requirement should not be extrapolated to cars without the advanced features. I don't see how a sane engineer would design a charging system that need registration to not kill a regular flooded-cell battery.