Thanks to a fellow board member, i picked up a slightly used set of M audio subs today, and installed them in my 00 M5 this evening. There are several threads on this for the details and how-to. This is a nice mod for those of us with an 00 car. I am not an audiophile, but to my ears, the improvement is dramatic over the oe nokia 5.25" sub box. The M subs may way a touch less too!
On my 6/00 production car, there were 2 of the 3 needed holes to mount the subs, and i drilled for the third. Really pretty straightforward, after finding something to hold the speakers up under the deck while i lined up and threaded the bolts from the top. I took apart the nokia sub box and removed the wiring harness from that, to avoid having to cut into the car's harness.
Overall, there is noticably more base, a nice tight sound that you can feel thru your seat. To my ears, it was certainly worth the 2 hours it took to install, and you can get the subs pretty inexpensively from someone who has upgraded to a "real system."