After being hacked several times we were thrown into the deep end of joomla site security and forced up a very steep learning curve.
We looked into each and every joomla security extension and settled on the Anti-hacker system, so far - we are very happy with that. Its a reassuringly complex piece of software that offers many different levels of security so you can refine it to your particular needs.
Initially, it wasn't trouble free setting up, however - the support team were very quick to respond and helped us through the entire set up and configuration process. They are also continuing they efforts to help us by looking into a conflict with another 3rd party component.
We particuly like the fact you can install the joomla version either on a shared server to protect many sites in one go or on as many sites as you like and set the security level accordingly for each site, a handy feature if you have different sites requiring different levels of protection.