For what it's worth, I've got one of BobbyM's sightglasses (brewhardware.com) and I really love it. It certainly isn't essential to making good beer, but I find it very convenient. I previously used a dipstick and found that annoying for some reason. That said, there are plenty of things other people love that I find kind of pointless, so there's no objective answer here.
If you do decide you want one, though, I highly recommend BobbyM's based on my very positive recent experience.