The only problem with this extension is the technical support that is almost non existent. Also, the documentation is not that good, but can be improved.
If you buy this extension, you are on your own if you face problems with it. This is the reason I give this extension 4 stars instead of 5.
First, the support. I know the developers work for free (this is a free extension) and have absolutely no obligation to do anything for you, but the support they provide is very bad at times, with VERY generic responses (when they respond to you).
Second, the plugin support. I use MyBB with Joomla, and this plugin (it's an official Jfusion plugin) was silently and "unofficialy" abandoned by Jfusion Devs. It works, but only some of the basic features.
Some of the problems: dual login don´t work both ways, no frameless integration, no discussion bot, no bugfixes (it has various bugs), no new features (already present in other official plugins), Joomla don´t work good as slave, etc.
Reading the support forums for answers and help, you can see that the developers only reply that they don't have a dedicaded developer for the plugin, so no work on it. You are on your on. "If it works for you, good. If not, well..."
Also, the plugin is for MyBB 1.4, with is almost EOL.
The documentation they offer is sometimes confusing, lacks information and is not up-to-date with the current version.
Jfusion is a good product, works almost out-of-the-box, and I recommend it, but beware that depending on the software you want to use with Joomla, You can end with almost to none support and help and your sofware be "forgotten" by the devs in a near future.