One advantage I can see is if your ISP's DNS servers are heavily loaded with users.
Then using the servers at OpenDNS could give you faster connection times.
My ISP is AT&T. I have noted times when using their servers that a click on a link would essentially be ignored, i.e. no reaction to the input, requiring using the refresh button if the requested address was shown in the address box.
I would attribute this to overloaded servers unable to handle the load from network requests.
I would assume it would work with cable, unless your cable company is using some odd sort of setup for their network. works fine with dsl.
Same dangers as using your ISP's DNS servers. It all depends on what sort of people are employed by the company.