I had GB on my SGS2 and as per official upgrade thru Kies I upgraded to ICS.
Personally, I think GB was better...
1. The major irritant I see is, that from the call log, in ICS, you cannot access the contact list. In GB there was a 4th icon on top which could get you directly to the contact list. IN ICS you have to get out of the log and open the contact list separately thus many key strokes for a simple function.
2. The favorites are now in tile format. I think this takes too much space and thus you have to scroll down to find the correct contact.
3. One thing I have failed to understand, with so much of disc space available (I am using 10GB or 16GB available), why the call log is only a couple of days old. Why is it not possible to store a longer call log. Before I changed to Android, I was using the Nokia communicator where I was used to a virtually unlimited call log.
4. Similarly you could use the filter option on the communicator to filter the log or even the stored messages for specific contacts or time. Here you have none of these features even though this is supposed to be much advanced.
5. I have not seen any significant improvement in battery life
6. The data settings now include IPv6. This is good but the problem was the phone automatically switched to IPv6 whereas not many operators have implemented this. It took me 2-3days to understand why my data stopped after upgrading to ICS. Changing it back to IPv4 it worked.
7. I am yet to see the new data usage function (since I have not reached my limit). I was earlier using Onavo app for the purpose which was very efficient.