ICS is less efficient than Gingerbread, I've noticed.
-There is equal "warming" of the device on both, something which should've been patched with ICS. Its a kernel issue, AOSP roms don't suffer as much.
-GB goes into Deep Sleep nearly instantly. There's so many things interacting with the ICS kernel and keeps it "sleeping but running background tasks". Issues are only now getting sorted.
-Battery life is great with GB, not so much with ICS: deep sleep issue, possible memory leakage.
-Stability ofcourse is near perfect on GB, though quite a few annoying little quirks with ICS ROMs

... is it still worth upgrading?
I'd say yes. The little issues are nearly all kernel issues (Samsung rushed ics for the note, while they concentrated hard on ICSgS3). They are getting identified and fixed, and these issues don't affect all users.

Anyways if you are coming from stock ICS (samsung) here's the files and guides:
-Download a TouchWizz-based kernel that is SAFE. It must be for Odin (tar file).
Franco r5, SpeedMod K3-5, or GL_NOTECORE (New!).
-Download Odin v3.185
-Download and Install Kies on your PC, or find the "USB Drivers for Samsung Devices".
Back-Up Phase #1:
-- Back up all your media to your PC (movies, videos, photos, pictures, documents, music, voice recordings, etc etc)
-- Back up contacts (Sync to Google Services, or use the App), Back up sms (install to phone), Back up Call logs(install to phone), Back up APN-optional-(install on phone).
-- Copy the files created by the Back Up Apps to your external sdcard/pc

- Now you put phone to Download Mode (Hold Home, Vol Dwn and Power)
- Quit Kies, Run Odin from PC
- Insert cable phone<>pc
- Odin should identify your phone (Com something comes up, first box lights up Yellow)
- Click on "PDA" and select your .TAR file (which is your safe ics kernel)
- Hit Start, and let Odin do its magic.
- Your phone will automatically reboot (takes 2 mins), if it hangs on bootloop remove the battery and turn phone on normally
- Everything should work and you should NOT have lost your Media, Apps, Personal Files (contacts, logs, sms, apn).
- You should have "Superuser" or "SU" App installed
- You may have "CWM" App installed too, otherwise you should have custom recovery without the app
- Install Titanium Back up (Allow "Superuser" rights)
- Backup your Apps with Data with Titanium Backup
- Keep a copy of this Titanium Backup folder, the Back up files (calls etc) in your PC/external sdcard
- Back up your EFS by downloading the "EFS PRO v1.8.3" and running it on your PC
- Finally you want to enter your custom recovery (Hold Home, Vol Up, Power)
- Then make a Nandroid Backup
- Keep all these files safe in your sdcard and PC (nandroid backup folder, EFS file, Titanium backup folder, files from Backup Apps for call, contacts, sms, apn, any other personal/media files you treasure)

First time, total process might take you upto 30min hour (reading, understanding, following step by step, double checking etc etc). Afterwards, whole process would take you 5mins tops.

And after that, if you want to experiment further etc etc revisit this "To Do List" I posted previously, starting from Step D.