Congrats for Weka.IO. They managed to pull it through after the CEO visited the DConf and asked for help.
D is a brilliant language. I got drawn into D by this article[1]. I've used D to replace some of the test tools where Java/Python were used. GC is a friend in such cases. I could've written them in C++ as well, but Phobos wins over STL hands down!
Initially the "solution" test application for one of our REST services was written in Java. It covered the functional, performance and longevity tests. The REST service itself was written in C++. Ultimately the customers reported that under heavy load the response time of the REST service is too long (> 400 ms) and we fixed the code as well. But during tests we were unable to really load the application to the fullest. 48 cores & 48 threads in Java and we still couldn't overload the REST service. We had no way to prove that our fix actually worked. Ultimately, we rewrote the test app in D and boom, we were finally able to achieve the load that customers created in their production env - both in terms of response times and throughput. I could've written it in C++. But D's std.parallelism + template mixin is a boon, thanks to David Simcha. Based on our tests, the D test app was 20 times faster than the Java equivalent (written by one of the finest and the expert).
Sure, there is a lot of visual noise due to @nogc, etc, etc. But beneath that there is a beautiful and readable language that's waiting to unfold, if and only if the D authors realize that they stop the feature creep and revert to sane defaults.
On the other hand, I started to convert the REST service from C++ to D last year and it didn't go well. Blame me! There are _very_ rough edges in D as well that are result of over engineering. One such thing is the const'ness in D. It make it impossible to use const, ever, by library authors. I don't care to go and fight in D forum. Manu has already screamed many a times. Lately I found that one of the reasons for poor performance of my D port was also because of auto-decoding! Duh! It was not even GC. (The default should be fast, isn't it?). I spent a considerable amount of time for this port and eventually it failed to meet the performance vs maintainability trade-off. C++ is getting better year by year, at least every 5 years :-). std::vector, std::map, etc are safe for concurrent reads (at least on POSIX). std::vector has erase with iterator. Phobo's std.array doesn't have an equivalent features, albeit it is much more readable than STL.
FWIW, D doesn't deserve the negative publicity that it usually gets. It's a fantastic language. Try it and if you like it, enjoy using it for the right use cases. After all choose the right tool for the job at hand.
[1] https://wiki.dlang.org/Component_programming_with_ranges