Thus the "somewhat." Still, I've downloaded everything from torrent clients to open source video games from SourceForge, and many of those projects have (or, much more likely, had) active support forums on SF. It's a shame that they haven't pursued that route further and instead are just going after the Github/Bitbucket/etc market.
It's a lot better than github, IMO. Still not as good as a real project page with binaries, or ideally just having your package in macports, mac app store, ubuntu/debian repositories, etc.
I dunno. I would rather have a page with binaries plus macports, etc. There are a lot of cases where you may want to reach out to markets not covered by external repos or where you haven't gotten into every external repo you want to.
Yes, you should have both. Plus ideally presence on GitHub and Sourceforge. (I also would never use a service as my only repo...)
(We got into Debian/Ubuntu repos not too long ago but there are still many more to get into.)