There are a lot of membership sites running aMember. I do have a site running it but sorry, I am not prepared to share the url.
Some of the other scripts, especially the newer ones require PHP5. If you already have a site running on that particular server you may need to consider the incompatibilities you may encounter if you upgrade to PHP5.
Amember allows you to protect any folder/s already previously setup on your server. Other scripts do not or may not allow that.
Yes, aMember is expensive compared to the others which may be free or cost only $17 but its worth it.
When you purchase aMember, they give you a 3 months temporary license key. After 3 months, if you do not ask for a refund, you will then be issued with a permanent key. It can only be used on one domain although you may protect any number of folders you create.
If I recall correctly, Memberfire protects a default folder it creates. I did explore it further as I had problem installing it on my server.
Download the trial copy of aMember or search for a copy in here and give try. I am sure you will find it easy to install and the features are better than the rest.