I've heard many people claim to have a drop in Adsense when turning on the HP ad program.
I was given the impression by another hubber that the HP ad program, once shut off, will deactivate at the end of the month, but I am not sure if this is true.
I am still experimenting with Hubpages but I have had success with my own sites in the past so I will probably leave their ad program on for a month and see how it goes before deciding to shut it off, but the going conclusion seems to be that the HP ads don't target keywords as well as Adsense and take away from your Adsense earnings.
Zubair above me has a great point. I think if the majority of your traffic is being generated within Hubpages itself then you should use their ad program to make money off views, but if you are trying to build traffic by ranking in search results then you might be better off allowing Adsense to run it's course alone.