Apart from allowing the owner the ability to check out a number of networks, there's a couple of other examples of why you might wish to IMEI unlock your phone even before your contract ends. Those who are going abroad for instance, you'll probably decide to take advantage of a domestic sim card whilst you're there to avoid heavy roaming charges. Maybe you've decided to sell your handset then you'll probably get additional money and much more interest in a smartphone that's free to use with just about any carrier.
Absolutely nothing is unlawful about unlocking your smartphone. It is your phone and you're well within your rights to use it with any carrier you want. But, there's a chance IMEI Unlocking your phone may end your warranty, so it may perhaps be best to hold off until you have reached the end of your contractual obligations before you make a decision to get rid of your existing carrier.
When your mobile phone is unlocked, you will be ready to locate the lowest priced cellular packages out there - a sim-only deal. These usually work on a rolling month to month basis, which means you can utilize the package provided you are content with your smartphone or you can up grade when a sparkly new device comes out or your outdated mobile ultimately craps itself.
If you're not sure who you can trust, I highly recommend Unlock.Zone. I have used them personally and was really happy with the service I received, and ultimately that they successfully unlocked my cell phone. They provide unlocks for all major brands and carriers, offer a guaranteed price match promise, but first and foremost, have a full refund policy in the unlikely event they cannot unlock your smartphone.
Here's a link to their home page: Cellphone Unlocking Service