The software has been very easy to learn, it's user friendly interface makes configuration/setup fast - especially when you have someone else helping with your configs! It can be hard at times learning all aspects but once figured out its great! There are many different ways that this program could work better.
For example I would love if there was more options in terms of adding new domains as well as having some sort of "add-on" system like cPanel does. Its really good product overall though we've seen almost no problems or issues with using it over our years running websites. We use ISPCONFIG primarily to add email accounts (Gmail) & web hosting services from GoDaddy.