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Yes. Thanks for asking. I'm doing this Dictionary series and it is important to know what are the most important info. I'm answering here and adding to the text as well.
Code is closed, yes. Written in Java, Javascript and C++. Client is open sourced but the server, called NIS is closed source and binaries obfuscated. Totally closed. The team though is well known, you may simply look at the site. But that does not mean transaparency, right?
Cheers.
@bitworkers

Wow thank you. That's really important information to me. I can't get behind a crypto-currency that is closed source. That's madness.

I totally agree with you. It is simply non-sense trusting such an enoumous endeavor in the hands of a few. I wonder what are these devs thinking??
Att,
@bitworkers

Well, as you said, majority of code is open, just some parts are closed. So technically you can say it's partially open or partially closed. As I understand they are opening it step by step as time goes so one day soon 100% of the server will be opened. But I agree, at least network should be 100% open source.

It only takes one line of closed source to steal all.

Correct. :D Depends on what you believe now. I don't believe it will be opened at the service end. And many do not even know what that means and are buying low to sell high.
@bitworkers

Thing is that NEM team has proven to be very honest over years, so I have quite confidence even though not everything is open. This 100% open-source demand is bit overrated anyway. On one side it allows code review, on the other side it's also provides better possibilities for searching vulnerabilities to some bad people. Thing is that unless you are completely paranoid you are always using some disclosed code on your computer. So there is always some default trust and certain risk. Even if NEM is 100% open-source, you usually have other closed software on your computer that you use and even if not, your hardware is almost 100% closed source. Who knows maybe current state is the best compromise right now in this imperfect world. At least from my point of view, I can live with that.

Your experience comment here is so helpful really for many of us crypto-novice that look up to experienced comments like this to make trading decisions. Nem does look good in general looking their volume and gosh, the recent spike!

Thanks for commenting @jimmco
I believe bitcoin has shown it is possible to go open source all the way and even more, it is necessary. But I do respect your point of view.
Cheers,
@bitworkers