Jailbreaking you iPhone can have a lot of benefits. From sim unlocking it, to just adding some games not in the App Store.
It is a pretty simple process, and with a little patience you can have your phone jailbroken in just a few minutes.
Jailbreaking is completely legal, and 100% reversible. Apple is not very fond of the process, but again it can be reversed and no one is the wiser.
So theres a newer program out there that can jailbreak you iPhone 4S in under 10 minutes with almost no prior knowledge necessary. plug in your iPhone, open the program, and click the button and follow the simple instruction it displays on the screen. The program is called Absinthe 2.0 and it is available as a free download for Mac, Windows, and linux. i have only tested this for the mac so far, but so far so good, very simple, easy to use and pretty quick process.
Absinthe 2.0 does support other iDevices, so check it out and enjoy your newly jailbroken toy.