I have a Windows Server 2008 R2 server at home right now and I am ready to move on to a non-Windows server, so Zentyal was a welcome surprise for its ease of setup, Active Directory, and roaming profile features.
I had the same question you have an contacted Zentyal support about purchasing a commercial edition license for home use. I received a response that I was very pleased with. After considering paying for Windows Server, I was pleased with the price. I haven't taken the leap yet, but I think I will after I have some more time to test it.