The negatives first. Security Images - Tricky to implement as it requires lots of core hacks. Each time you upgrade Joomla, those hacks need to reapplied with new hacks suited to the new Joomla version. The captcha may often not show due to template overrides so after all the installation work the captchas may still not show. The captcha is no longer available for the administrator page so you can only have captcha on login/registration and contacts. The backend is a bit hard to comprehend and Walter's English translations need some assistance from a natural English speaker. It is also not easy to make all 3rd party components accept captcha, each requiring specific hacks to code. Support is not available for the older versions for security images. Walter's website is as confusing as the component backend. The documentation is thorough though confusing at times with unresolved ambiguities. Walter's components and acoutrements seems a little chaotic sometimes.
You have to be brave to attempt to install this for all the above reasons, but if you make the attempt, the end result might well be exactly what you want.
Good points - Good and configurable captcha. Walter does his best to handle innumerable forum support requests. The joomla 1.0 version provides captcha on the administrator back end as well as the front. The product works. Walter is trying very hard to provide a product, documentation and a support level that is better than most.
If older versions were better supported and the back end of Joomla 1.5 was better protected I would give this component an Excellent.