Are E46 M3s still a good investment or has that bubble burst?
I'm no expert by any means but have been loosely watching the market for the last few years, and have noticed that the good ones are holding strong and or appreciating slightly.
If you can I'd buy a manual face-lift with good history and factor in the cost of a sub-frame repair you'll probably not go far wrong.
As far as an investment goes, and I appreciate you've probably used that term loosely anyway, I'd say they're not going to be moving much from here on out.