On the z3 with m52tu 2.8 which shares the head and vanos system with all m54 based engines I had a wavering idle problem at part warm which turned out to be vanos seals. My car sat for years doing very little which probably caused it. In my opinion if it starts from cold and over the course of a few min settles down to a stable idle AND you have no loss of mid range torque.. good to go.