Does anyone use ohloh.net for their open source projects?
Clearly yes ... though not necessarily in the sense that you mean.
Is this a good alternative to Github? Or are they completely different?
They are rather different. Github (and SourceForge, GoogleCode and a few others) are more or less "full spectrum" project hosting sites.
Ohloh.net is more a social site for open source developers. It is a good place for finding out who is "big", who contributes to what projects, and what projects are valuable to the community. The project download service is secondary to this.
You cannot host a project on Ohloh because (for a start) there is no facility for code checkin.