Actually, I have no problems with Yahoo. Save for the recent server tweak that cost me 200+ members, I have no problems with Yahoo, especially the ones you discribe.
As for spam and groups ruined by it, well, I blame irresponsible moderators and owners for that. You won't see any of that shit on my group, ever. People have the tools at their disposal to deal with spam so the spam never sees the light of day, but they aren't utilizing it. So all the negative ramfications that come along with spam is all their fault. You can't so much blame the spammers. Afterall, they are being paid a commission to advertize to you. They post the messege and leave. They don't stay to read whatever it is you have to say (which is pointless and not worth your time to respond anyway). They are just like telemarketers, and just like telemarketers you have tools at your disposal to get rid of them.
I've been using Yahoo since I came to this community, and its been nearly two years, if not two years. Yahoo has been good to me, and I'm using the free service.
It probably boils down to your internet connection and it's reliability. I for one am FOR what Yahoo is doing and has done (for the most part). And with the recent upgrade, its better than it was before. As far as the member count loss, I'll eventually regain those losses provided they join again, and why wouldn't they?