iPhone sales really took off in the Summer of 2008 when the iPhone 3G came out. Then another huge boost came in the Summer of 2010 when iPhone 4 was released. In between we also saw the upgraded 3G, known as iPhone 3GS, but the sales didn’t increase as much as when the “3G” and “4” was released in the Summers of 2008 and 2010.
Depending on where you live, iPhone contracts varies in length. In the US, I believe they are two years. So you’re assigning yourself to the same phone company for two years, unless you pay a fee to get out of the contract early.
Luckily, in my home country (though I don’t live there any more…), the maximum allowed contract length is 6 months. So when you buy an iPhone, you pay more for it than what you would do in the US – but at least the choice of contract is yours after 6 months, so you have the option of switching to a different carrier with better prices or coverage.
You could also look for iPhone SIM card plans that doesn’t require you to buy a new mobile device, in case you’re happy with your current iPhone – or whatever smartphone you might have, of course.