Personally, I don’t install custom modem on my Samsung Galaxy S II. Usually, I will stick with the modem that comes along with the firmware. However, you might love that particular firmware but wish to upgrade the modem only. So you have to install official custom modem on your Samsung Galaxy S2 manually. What is the benefit of using custom modem? Well, one modem version does not work well for all the region’s telco. So there are several versions. Some modem might have better signal, speed, reception and coverage and some don’t. Choosing the correct one take time. You need to benchmark the 3G speed, battery consumption, signal strength… Monitor its performance for days. So I don’t change modem much unless I found it really bad (which I did not see one yet).