Haven't done it on the X1, but did on our previous 135i. General summary is:
- Much smoother, softer ride
- Quieter in just about every condition
- Less tramlining
- Much softer sidewall feel with mushier turn in
- Exposes just how soft the rebound control is and really makes you want to change spring rates
I was happy with the change and if you're looking for a better ride and aren't concerned with giving up a little bit of feel and sportiness, it will be okay without additional changes. If you want to preserves some of the sportiness, you'll likely want at least new springs, if not new shocks too.