I know there are many improvements and bug fixes that would make me sense to upgrade from X3 to Plt but here I'm sill running happily my X3e. It runs smoothly and I have learned to live with anomalies.
Many of the new features aren't targeted for me. For example:
Anderton's guitar stuff sounds nice but I record guitars and basses thru Axe FX II. Guitar Rig 5 can do the rest.
Control Bar's Custom module: no need - I have my own utility for that...
Mix recall: this is some serious stuff but I wait until it's tested to death.
Addictive Drums: no need - I have NI KOMPLETE, tons of other drum kits and ability to record real drum kit anytime.
Ready baked loops etc: no need (genre mismatch).
I'm not a typical SONAR user. I use SONAR to record and mix bands but also to produce songs.
I feel SONAR is mature enough software and now I can wait longer before upgrading. It hasn't been the weakest link in my studio since X1 days. It's not hardware or software that is the weakest, slowest or buggiest link - it's ME. :)