JMS Multisites works right "out of the box", but you must be familiar Joomla and it's configuration. If you do not know how to configure JMS or a particular feature, there is a very informative FAQ and forum on the JMS website. Edwin consistently monitors this forum and answers all questions for free.
If you need advanced support or an advanced installation of JMS, then there is a paid-support feature that Edwin offers his customers. This support is well worth the money and time if you want to create an advanced multisite-capable website.
I was able to create "slave sites" on my own without support from Edwin, but the features I wanted on my website required advanced skills with server-side scripts and code. I was not familiar with this code language, so I paid for Edwin's support. Again, this support was well worth the money and my site is now set up the way I have envisioned for a long time.
The people who have posted negative reviews of this component obviously don't have any basic knowledge of Joomla; otherwise they would have been able to successfully implement and use JMS. Even if you do not have basic knowledge of Joomla, simple investigation into the JMS forum will help you get things up and running.
Edwin, thank you, thank you, thank you. You have gained a long-time customer with me! JMS Multisites is great, and I highly recommend it for any website needing multiple instances of a Joomla installation!
Great job, JaggySnake!