We've used Securus for about nine months and I'm impressed with the product. Yes, it isn't cheap, the interface is clunky and the upgrades have been poor, but I can't fault the company for the level of their support. What other software vendor rings you regularly (about once a month) to check on problems and provides immediate response? Their posts to this board are consistent with their interest in ensuring the product works.
And as for the product? Students know I can see all their misdemeanours and they can't hide a thing. Someone looking for porn? Easy to identify, document and deal with. Proxy bypass sites? Well, let the kids find them for me. I don't have to wait for the latest novelty, I can see it as it develops. It's part of the process of moving away from the "ban everything" mentality to "make them accountable".