Harmonium is just coming from a very unenjoyable FSX-experience, so I doubt he will go back any time soon.
I have both (or should I say all "three") of the 737NG's available. The old PMDG 737 and the Ifly 737 for FS9, and the NGX for FSX. I can only use my NGX with acceptable performance if I fly somewhere in Antarctica. And then it looks worse than my highly-tuned FS9.
To be honest, I don't understand what all this fuzz about the NGX is about. Yes, it is a great plane! Yes, it has more features than you can ever imagine and the resolution of the textures is amazing.
But, do I see all of that when I just want to simulate a regular airliner flight from Boston to New York? No. If I want to do that, I honestly don't care if I can click the circuit breaker on a panel below the carpet or if I can replace the light bulb with my spare bulb from the case behind my seat (you see, I'm exaggerating a bit here, I admit. But you get the point...)
When it comes to flight dynamics, performance data and systems, the Ifly in FS9 and the NGX in FSX are not that different. Ifly just released a free Feature Pack for their product, which adds a lot of functions like vertical profile display on the ND. So the difference to the NGX even shrank a little further. Both products are programmed with original specs from Boeing, so the difference in flight behavior is probably in the 0,XX% area.
I love my Ifly in FS9. Great performance and great system simulation. And I won't switch to FSX any time soon. I'm not "bashing" FSX, but I am very happy with how my FS9 looks and performs. I fly IFR 99% of the time, so I don't really care if I can see cars on the highways or if there is more autogen around.
See for yourself, I doubt that FSX would look significantly different in this situation (except for half the FPS).