While there are certainly qualities to like about this new X.9 derailleur from SRAM, there are a few to gripe about too.
I have been running the 10sp X.9 for 3 months now. It replaced a 10sp X.7 when I bent that cage bent beyond repair. Looking to upgrade to a D that was more durable but wouldn't break the bank, I landed with the X.9 on my tool bench. Initial impressions were good, it installed with no drama, and was painless to dial into proper tension and alignment.
The shifting performance of the X.9 felt just about the same, maybe a touch quicker, as the X.7 it replaced, which was no slouch. I suppose my complaint stems from using 9sp SRAM drivetrains from the mid/late 2000's that had a snappy, secure feel. The old SRAM drivetrain feel has been lost in this new generation. The new stuff feels more like Shimano than SRAM. In fact, if it weren't for differences in shifters, I am not sure if you could tell a difference between the two nowadays.
I have also found that this D is not as stout or stable as I would have liked. It takes very little misguided force to through off the derailleur alignment. In the three short months I have been running the X.9 I have frequently had to stop mid ride to inspect and adjust the D because it started skipping or shifting poorly, sometimes without external force to the derailleur. One hill it's fine, the next it's skipping through gears 4-6. Kinda peculiar and quite annoying.
Overall, the X.9 gets the job done in a stylish, lightweight package that is easy to setup. Like most people, I prefer as little fuss out of my drivetrain as possible. The less your D is on your mind, the better it's working. With the X.9, I find myself thinking about it's unpredictable shifting performance a lot and find myself giving quick little turns of the barrel adjuster here and there to keep things running smoothly. Kinda fussy if you ask me.