Let me backtrack perhaps.
About 7 months ago my radiator went at 92K miles. I understand this is very normal for BMWs as they use plastic radiators. I got it fixed by an acquaintance who is a head mechanic at a BMW dealership. He replaced the radiator, water pump, flushed the coolant and changed the thermostat.
I didn't notice a problem with the car until winter hit. The coolant light is always on now but the temperature is normal. It takes a very long time for the heat to come out. I'd say at least 15 minutes.
I took it to a rip-off BMW repair shop (the acquaintance is too far away) and he said I have a serious issue from just my explanation. Since this car is literally falling apart on me (ABS is gone also), I'm not looking to invest more money in it because it likely won't pass inspection. If it is something like bleeding the coolant, I may take it in but if its something serious, I may just hold out. From everyone's explanations, it doesn't sound as bad as the repair guy said.
FWIW, when my radiator went, I did drive the car with the temp gauge way in the red several times. Most of the time, it wasn't for longer than 15 mins. I don't think I damaged the engine since the mechanic didn't find any problems with it. Just from listening, the engine sounds great.