First off I do not know these guys so this is not a fake testimonial.
I found out about J!WHMCS in the WHMCS forums and took a look at the product. I ended up purchasing the leased version to give it a try and the amount of money they charge for the leased version this software is a steal.
The automatic install works great for a this robust software. It is not too complicated, if you can use WHMCS you can use this. I encourage EVERYONE to completely back up not only your databases but the entire install. It is always good to start fresh with your software even though it is not necessary.
The support that I received from Justin and Steven was fantastic. I bugged them quite a bit because I fudged my install the first time.
It can be a little sluggish if your server is with a poor hosting company so make sure things are good on your end because Joomla itself is resource hungry.
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