Android 2.3.4 on modem version I9100UHKG4 sucked severely on my SGS2. My data connection would get stuck uploading data without ever getting data downloaded back. The upload arrow was the only one lighting up. I needed to turn data connection OFF and then back ON to get internet working again.
I upgraded android to 2.3.6 and it came with modem version UHKE2. It worked like a charm, 100%, not a single time I had the problem I was constantly having before.
Two days ago I upgraded the OS to the Polish version of the 4.0.3 (ICS) and now I'm getting problems again. I'm getting "no service" after a whole day so I'll downgrade just the modem to the previous version. The current one is XXLPQ.