Mate has X0 on his bike and I've ridden around a bit on it, but I prefer my XT.
Definitely. I run X0 and I've had nothing but grief with it. Great when it's working but more often it's not and my SLX on the HT runs better.
I started with an X0 3x10 and changed over last year to a 2x10 42-28 after I deliberated for ages, posted on here and then decided I would need the bigger ring more. The change was for the better but I did notice straight away that I missed the granny to granny more more than I needed the big ring. The other thing I noticed on the 42-28 is that you shift between the front rings a lot more as it's simply a case of big front ring for any kind of downs and small front ring for any kind of up. Ran that for a year, lived with it and liked it. Even enjoyed working harder on the climbs - I think !
I've just changed over to a 39-26, mainly because the half decent chain devices are only compatible up to a 40T ring and I desperately needed one (and when I say decent I don't put the MRP 2x in that category - bag of sh1te, avoid !). This new ratio is miles better, I didn't realize how much I'd missed the smaller ring and that shifting from front ring to front ring has gone. Now it's small 26T ring for the severest of climbs IE hour long in the Lakes, but for quick up and downs I stay in the 39T.
Not sure about the XT or XTR but definitely needed a new FD for the X0. Even needed a new one when I changed over from the 42T to 39T. The FD sits too high and the chain tends to bounce off the big ring down to the lower mid decent, with the right FD it offers some chain retention.
Have you thought about your rear mech as well ? If you go 2x you can put a shorter mech on and pull it out the way for more clearance. Thebeard has highlighted another ferkin problem with the X0 with the rear mech pulling too far back if you go small small (but how often would you do that). You don't have this problem with the XT or XTR and you get the added bonus of the clutch - I know someone who couldn't live with the X0 problem and changed over to XTR and it sorted it out no problem.
One final nail in the coffin for SRAM. They have **** all in stock when you need it. Not even their most popular rings at the beginning of the season !