I don't think PowerOn is a shady company. They do, however, need to change their online estimate system--though if their estimates were more accurate (true to their final offers) I wonder if they would get enough business. After reading through these reviews (which I wish I'd done before sending my computer to PowerOn) it seems like, in the end, few people receive the full amount of the online estimate. Many do not receive an amount that's even close. For instance, the online estimate for my MacBook was nearly $200. After I sending my computer to PowerOn, they said it was only worth $60. I considered my computer to be in "good" or "fair" condition--it worked well and I would have contintued to use it, if I hadn't received a new one as a gift--but PowerOn said it was in "poor" condition. Since used MacBook chargers sell for nearly $20 on Amazon, I find it difficult to believe that the rest of my working, albeit old, MacBook is only worth $40.
Bottom line: If their estimate system was more accurate and I had known that PowerOn was only going to give me $60, I wouldn't have sent my MacBook to them. But since they already have it, I don't want to risk having it shipped again, and ... well, I guess I'm just a bit lazy... oh well, I'll take the Apple giftcard. But when my new MacBook gets old and it's time to sell, I'm going to sell it myself.
I say, if you just want to get rid of your old computer, phone, etc., and don't care how much money you get for it, definitely use PowerOn. If you actually want to get what your computer is worth, or anything close, you should delete all of your information from it, and sell it through Craigslist or Ebay.