Review on Enhanced Keyecu EWS Flip Remote Key 315MHZ ID44 1998-2005 for BMW 3 5 + 7 Series HU58: Upgrade Your BMW Key Experience by Runbi Alvarez

Used on my BMW E39 540i works great all you have to do is initialize the key. He'll sink with your car and do everything your original key does, like program key transponders. So you can start the car or switch your old transponder to this one and cut the key. Also remember you need to make sure you choose the right frequency and key style, look at the back of your old key and check the frequency and adjust it, for example 315MHz was mine and mine was a HU58 key. They have another, 433MHz and HU92. Just make sure it matches yours or it won't work. But if you just want to use it as a remote to open/close etc, it still works great in case your key has a horribly dead battery.