I mostly browse the following forums not detailing related:

6speedonline
bimmerfest
bimmerpost/2addicts
n54tech
garage journal

I`m not an active poster on any of them. But I definitely learned a ton from 2addicts when I was looking at upgrades for my 2 series.

I love going on 6speedonline market place and seeing 95% of the cars I can`t afford lol