Not the greatest in hot climates (Australia) I have to say. If my M3 has been out in the sun and is quite hot (>30C), the seats get quite hot too.

Once you're in the car even with the air conditioning, because the leather can't really breathe you end up with a pretty sweaty back...

The newer cars seem to have slightly perforated leather which seems to help a bit. The newer air conditioning units do a bit better too. My 98 model struggles a bit with the Australian weather even after it's been serviced - the climate control has issues understanding our weather too.

Given you are from the UK though that is unlikely to be too much of an issue for you....