I used to have a x3400h with Audyssey App on my iPad to do the fine tuning.
When my speakers were Focal Profile 908 (really capable speakers but not incredible), I heard a real difference with or without Audyssey.
Now I have way better speakers (Triangle Magellan Duetto) and the difference is not that big on the front but Audyssey help having a coherent sound with my center (Focal CC1000) and surround/height (Focal Dome) speakers.
6 months ago I bought the x3800h (it was a nice opportunity, I bought it only 799 € and sold my x3400h 500€). I bought DL Full Bandwith thanks to the Black Friday sale and ran it with my Umik-1 mic. I think I was expecting to much because of everything I've read about Dirac.
The sound is great but not so different that the one I get from Audyssey App. Of course it's much better than Audyssey without the App because you can't tweak anything without the App.
So now when I listen to music or when I watch movies on day time, I use Dirac because I like the explosions a little bit more (and it's possible I want to think my 250€ are not totaly thrown down the drain) > I should do some REW measurements to show if it's true.
For movies at night or watchin TV I use Audyssey because Dynamic Compression does a really great job.
It's too bad the Denon "Dynamic Compression" feature (compatible with Dirac) is not really effective.
I will not buy DLBC because it's way to expensive but I'll read the ART review / comparison with DLBC with much interest when it will be out on Denon (if the x3800h get it).
In conclusion
- I think Dirac might be useful with good but not great speakers and if you live in a house (not a flat with neighbours).
- Audyssey App on iOS/Android is a bargain and if you don't achieve good results, I think you should re-run it and be careful with the mic : if your 7 & 8 measurements are too close from your wall, it might kill the bass too much.