what a lot of folks do is get a guidebook.  I think Leland has an excellent one.  Then they get an idea of the character of the river from the guidebook, then add in the rating system (III-IV in this case), and they go scout it, get beta from locals or other paddlers out there, then run it or not.  I don't care how complex of a ratings system there is, none of them will be comprehensive, and anything more than the basics is a novelty, in my opinion.  Where is the sense of adventure in paddling, anyway?  I don't want to see a picture of every single drop on a run before I run it.  I'll know if I want to run it when I see it.  If it can't be portaged, I get that info beforehand and know what I'm getting myself into before I do it.  Unless i'm the first one to run something, that info is widely available and pretty easy to access, as well as infinitely more valuable to me than any subjective ratings system.