Really liked X1d, but am liking X2a even more. The enhancements are brilliant.
There is a lot of negative feedback on here, but it is human nature to only comment when things are going wrong. Many people are finding X2a stable, and love the developments.
Off the top of my head, some of the reasons to upgrade would be:-
Auto track zoom
PRV improvements
Take Lanes
Automation lanes
Console Emulation
ProChannel FX Chains
Various workflow enhancements, like right mouse clip selection
Breverb
I'm sure there are many I have forgotten. Some of the above points would justify the upgrade on their own IMHO.