I've replaced mine with a Behr expansion tank and had good results. It's at least as reliable as the BMW OEM tank, and costs less as well. Some people will advise sticking with OEM, but since the OEM parts don't last very long anyway, what's the point? LOL
My advice, if you're going to replace the expansion tank, now would be a good time to replace most of the soft parts of the system as well. Hoses, belts, and possibly the coolant level sensor as well. I broke mine the first time I replaced my expansion tank and ended up driving around for a couple of weeks with a low coolant light until my new sensor came in.
ECS Tuning sells some pretty good kits that are pre-assembled with various levels of parts. Or they sell individual pieces if the kits don't meet your needs, wants, or budget constraints.
Hope this helps!