How Do You Rate The Quality Of Traffic?

When it comes to different types of traffic, how do you personally rate what type is 'good' and what type is 'bad' do you base your opinion of traffic sources on your own experiences or how different types of traffic is rated industry wide?

For example, i know Basschick loves the TGP game and she says that traffic converts exceptionally well for her but, lots of others say that TGP traffic is some of the worst there is for quality.

Likewise, i love my organic search egnine traffic, but have a few friends on ICQ who tell me that i shouldnt waste time/energy in building se traffic.

So with so many people having differing opinions on the quality of certain traffic types, how do you personally judge whether one source of traffic is 'better' than another?

Regards,

Lee