It's mainly the rumors the owner has spread about it. The problem is the person now owning rights to it is not the original coder - the result is he doesn't actually know much about the script. It even goes as far as claiming to be "worlds fastest script" and yet, like almost everything else said about it, there's absolutely no facts to back up all the hype.
I'm using PHProxy, I think it's better than the others. So you have to decide If you hard to choose, just have 3 proxy sites and use 1 each. then you will know which is better..
Yes sure it does support logins. PHProxy is more stable and use less server resource than the others. Not so clear, but I heard about that.