If you search the forum you'll find a very nice discussion we had about the use of Starforce and how is the "correct" implementation of it.
Splinter Cell 3 took almost 8 months since its release to the market to be cracked and it was using Starforce (not only the packer).
If you deeply implement the Starforce protection in your game, you may be "safe" for a couple of months.There is a small-to-big performance hit (specially at the startup of the game,to much loading time), depending on the protection tricks you're going to use, but you don't have to worry much about it.