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Factory

Power Seats - Reasonable.
Heated Seats - Nice when it's below freezing outside.
Power Roof - Fine.
On board Computer - "Distance to Empty" is very useful.
Leather Upholstery - So what.
Wood Trim - Yippee!
CD Player - Useless, it skips almost daily.  Speed sensitive volume was nice however.
Hardtop - Sorry to have to put it on, but who wants road salt on the soft top?
Wind Deflector - A "must have".

Aftermarket

http://www.z3mirrors.com/ - It may save a life someday.
http://www.leatherz.com/mk2faq.htm - Very nice and simple to install.
http://www.kenwoodusa.com/product/product.jsp?productTypeId=20&productId=695 from http://www.crutchfield.com/