Hi Alesio,
I have tested Zentyal, ClearOS, SME Server, Univention UCS.
With Zentyal 4 the features for firewalling, like IPS, L7Filtering are very reduced.
I am using my SBS as firewall so I am very interested in this features.
ClearOS 7 has a Marketplace for extensions but not all are opensource/free. The GUI is imho not very good designed. At testing in a lab I got a lot of errors installing addons.
Univention UCS has a very nice and simple gui but you need a separate Domain Controller if you use the free edition. Paketfilter Management is only via command line possible.
SME Server has very interesting features but an very old gui.
So I came to nethserver which could satisfy all my requirements.
The possibility to use custom templates is a very great feature, which I have not seen by other distributions. So I can personalize my settings as needed and didn't have to reconfigure all these settings if I do a update. The GUI is very nice and the community helped me very fast!
If you have any further questions, please do not hesitate to ask.