i recently changed my transmission fluid. it took about 7 quarts and the mechanic said it was the worst he'd seen. he didn't think anyone had ever changed it. it has 150,000 miles on it. i was experiencing hard shifting and hearing a grinding noise when i changed the gear from park to drive or reverse. that stopped after the fluid change but it's starting up again. i've read that you have to flush it out 2 or 3 times. would that really be a good idea? thanks in advance.

Yes, the torque converter and passages will hold dirty fluid. Best bet is a transmission flush with a machine. It replaces 100% of the old fluid.

First off was the mechanic from the BMW dealership? When you are dealing with cars like BMW and Infiniti and Mercedes and etc. its really important to go to the dealer for service even if it isn't under warranty. It normally is good to flush it out a couple of times only if it has never been done before. I would make sure you are going to someone reliable though so you don't get ripped off!

I assume you changed the filter when this flush was done? Otherwise you will not remove any old particulates in the system. Did it smell burnt? Was it discolored? 150k is not huge mileage on these cars as long as it was not a tow vehicle. Generally, if you have only changed the fluid due to issues with the gearbox instead of doing it as part of regular maintenance, you might be too late.