I have used Bel Arc Advisor for years, I love it. I had a notebook that was so old that the sticker was worn off so bad that I couldn't read my Windows activation key. Bel Arc got it for me, note this will not work on unregistered copies. But if you have to reinstall Windows and can't read your key Bel Arc will save the day.
I've used Belarc rarely, like never, and for good reason. I am one of the few people who knew about it back during the days of Windows XP. Many people who ran Belarc uploaded their full diagnostic page to Google, or it somehow found itself there. Due to whatever configuration they had set up in Belarc, they were outputting a HTML page describing their entire system, and everything installed on it, including their WINDOWS XP product key. If you knew the right search terms to plug into Google, you could easily look up Windows XP product keys using this exploit. Eventually Belarc fixed this problem, but who knows how many VLKs, retail, and oem licenses were banned because of this issue. It must be good software, as many people still use it, but this problem alone, is legendary to me, as I witnessed my fellow IT friends demonstrate it firsthand. I cannot, in good conscience, give it more than an average rating, especially with programs like AIDA64 and Speccy out there. It will still do what its told, and with any luck, won't upload those precious keys in the HTML output ;)