The Challenge Tracker is working great for me -- much easier than trying to keep track of a post in G2G.
The one thing I'm wishing for, though: I'm doing the Source-a-thon and have found that if one person in a family is unsourced, it's likely that many of their relatives are as well. So after I use the challenge tracker to log my action, I want to go back to the person's profile so I can easily navigate to the next family member. I suspect this'd be true in many other challenges as well -- if one profile needs a particular change, relatives' profiles often do too.
Right now, I have to use the back button to get back to the profile that I worked on. I'd like the challenge tracker to either open in a separate tab, so I can close it and be back at the profile, or to include a link so I can easily click back to the profile.