I use it all the time. I knew someone who claimed to be a conservative, but opensecrets showed heavy donations to liberals, exclusively.
Another good one is the wayback machine at archive.org, if they track a website (like say John Kerry’s campaign website) it shows every change. If Wayback has the website, you can look for hasty deletions, shifts, additions.
That’s pretty cool! Sounds like opensecrets.org and Wayback machine might be the most useful things the internet has produced, aside from Free Republic of course.