I have been very happy with M42Club since Sheepdog & I started it. I originally made the "club" in a thread on Bimmerfourms' E30 section. Some E36 guys had started one in a similar fashion, but had said "no E30 owners allowed." So, I made the E30 thread to spite them, it attracted a lot more attention than I expected, and then Sheepdog offered to host the club on his servers. The rest, as they say, is history.
All along, I have been very happy with the maturity & respect shown in most posts. There have been a few flaming incidents, but most of them seem to be at least a few years ago. Generally, it seems that E30 M42 owners are a little older than the average E30 owner (except E30 M3 owners...given the cost & rarity of that car, you sort of need solid employment to own one). I think that the relative rarity of the 318iS, the fact that it is less powerful / "slower" on paper and it having 4 cylinders keeps a lot of the much younger guys away. Actually, now that the E36 has sort of become the Honda Civic of BMWs, it seems that most of the younger guys gravitate toward those these days.
E30tech has some good information on it. I guess it is good that I registered there in 2003 when it was still being actively maintained...although I have managed a whopping 11 posts since then lol. R3v also has a lot of really good, knowledgeable people on it. While they don't have as much M42 specific stuff, there is a plethora of generic E30 information available there (headlight upgrades, steering rack swaps, engine swaps, etc). The M42 used to get a LOT of disrespect all over the place. While some people still look down on it, the overall sentiment toward it has improved a lot in the last 5 years. In general, I would say that it is a well-regarded motor in the E30 community. There are so few of them, compared to M20 cars, that most E30 people never get to drive one & make comments based on assumptions.