Those that know me know that I am definitely a fan of Shimano XT and XTR over SRAM X9 and XO (LX v X7 is no contest...). I'm still rockin' the M952 XTR shifters and derailleurs that debuted in something like 1996! It's not that I'm hugely loyal to Shimano, though. In fact, I started out my mtb career with Suntour and I still think that XC Pro was one of the greatest mtb groups ever. No, it's not allegiance to Shimano, I just like the fact that it works, it's durable, and... well, it works and it's durable. That's pretty much what I'm looking for in parts. That, and it's light and good looking, too. There are some design elements of SRAM drivetrain parts that I am not really in to.
I just got some of the new (2007) SRAM X9 bits in today. Some of my feelings may start to change. First off, I've been spending a bit of time lately on the Turner Flux demo, and that has a complete SRAM X9 (2006) drivetrain. It works well. I've had to fine tune my finger muscles a bit to compensate for the different shifter, but so far I've been pretty happy. About the only thing I've not been stoked on is front (down) shifting under load. There have been a couple of instances where my XT front der would've done the trick, but the XGen did not. But I digress. Like I said, I just got some of the new '07 X9 in. They've changed a few things. First, the shifter gets the multi position mount, much like XO. Definitely a sweet feature for those of you out there that just can't get your shift/brake lever positions 100% dialed. The downshifting lever (thumb) definitely has a more positive snap than the previous version, which is also a good thing.
But the rear der is something else. I think they've finally got it. My biggest beef with the X9 rear der has been the cheap steel plates on the back of the derailleur body. I recently came across one that had been bent slightly enough that, even though it still shifted well, the low limit screw was no longer touching the innner der parralelogram plate, so the bike would easily shift into the spokes! The new 07 version addresses that. Gone is the cheap stamped steel and new is a finely crafted hunk o' aluminum. SRAM also changed the inside/outside orientation of the pivots on the der mechanism, so it should feel a little stiffer in the der shifting under load.
The 2007 X9 from SRAM may be the hot ticket for that level of drivetrain, and there's no question that XO works well. We'll just have to wait and see what that new Shimano XTR feels like ;) ...