Recently I upgraded my phone (Sony Ericsson Xperia Mini Pro) from Cyanogenmod 7.2 to Cyanogenmod 9. And you should upgrade too, if you are running Cyanogenmod 7.2. This is why:
The hardware keyboard layout can be configured from the settings menu.
The ‘symbols’ key on the hardware keyboard now actually works, so you don't have to switch to the software keyboard anymore when you have to enter an exotic symbol.
The dictionary on the software keyboard now works. Not that I ever use it, but hey, it might be useful for some of you.
The ‘dock’ at the bottom is more stable: it doesn't disappear for no reason, and the icons in it stay where they are.
Dragging icons from the app drawer to the main screen is a lot smoother.
CM 9 (based on Android 4) looks way cooler than CM 7.2 (based on Android 2.3).
But I went to some pain while upgrading the thing. So if you want to perform a similar upgrade, here are two useful tips.
The first one: Be sure to put not only the roms of CM 9 and the Google apps on your SD-card, but the ROM of the old CM 7.2 as well. If anything goes wrong, you can quickly flash the old image for having access to your phone again.
Second thing: before flashing CM 9 to your phone, you'll probably have to upgrade your boot loader. I didn't do that, and after flashing the ROM, the phone booted into a black screen; removing the battery was the only option. Maybe you can upgrade the boot loader with ROM manager, but I didn't try that. I just used fastboot (a tool that can be found in the platform-tools folder of the Android SDK), as described in steps 5 and 6 of the excellent documentation on the cyanogenmod wiki.
The new bootloader looks much slicker than the previous one, which was plain ugly :-). And when it's installed, you should be able to flash the roms for CyanogenMod 9 and the Google Apps for Ice Cream Sandwich. (But don't come after me if things break, because I'm clueless as always.)