I started with keyclone myself then moved over to pwnboxer, tried isboxer but at the time it seemed way to confusing for me i just didnt have a clue i was just noob at multiboxing, so i went back to the simple use of pwnboxer i found it great, did what i wanted it to just fine, it even gave me more FPS than isboxer.

Then i wanted to have wow on 2 computers and since you cant do that with pwnboxer i looked into isboxer again.
Yes it was confusing to me yet again but lax has been making some amazing updates to it that i simply dont see myself ever going back to the other two, its simply surpases them in god knows how many ways.

Plus the support you get from lax and others has just been amazing.

Keyclone is one payment
Pwnboxer yearly i believe
ISboxer every 3 months

This may seem off putting to some paying like that but what you get and the support you get will make your multiboxing experience a whole lot better.
Im still keeping an eye on what will happen with pwnboxer 3. Hopefully something really good.
Keyclone i dont even know what goes on with that or if it still gets updates.