Positive impressions. Yes, it may not do anything "perfectly" but it does everything really really well. Considering the price I am not sure I have ever read a single negative comment about JH5 on this entire website. I've had mine just over a year and they have taken a lot of abuse and still sound great.
I don't think these would be quite as good at the original price but right now I find them a very good value. They are also pretty light for customs but then again it is only 2 drivers compared to 3 or 6 like what I used to have.
It's hard to say when I haven't heard the 1964T in a very long time and no longer own them. However instrument separation is just fine with the JH5. It isn't like an analytical phone where that's the first thing you notice but I don't find an issue here.