"Unlocked" phones?

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Okay so my carrier is TMobile and I have been with them for approx 10 years. I was looking into changing my carrier b/c I wanted to save some money. After researching all the other companies out there are more expensive and not that many minutes a month then what I am currently ggetting. Okay so TMobile is now doing these plans where you can choose to "upgrade" your plan with out extending your contract which I love b/c it's cheaper to get the plan where you don't have to extend your contract. The reason why these plans are cheaper is b/c you don't get discounted phones. Which kind of sucks but I usually upgrade my phone every 2 years anyways. So it's not a big deal. But anyways.... I've been looking into buying a new phone so I won't have to extend my contract with TMobile but the retail price for these phone are ridiculous! OMG! So I stumbbled across Amazon.com which I love) and I guess you can buy phones that are "unlocked" so you can buy one of these phones and just insert your SIM card and your good to go. I seen some really inexpensive phones; compaired to what I was looking at before. but I am a little hesitant on purchasing one of those phones b/c I don't know much about them. I was looking into getting the Sidekick phone and I know TMobile sells them but buying these Unlocked phones are they really legit? Can someone please help me out? I just don't want to buy one of these phones and be a waste of money b/c it didn't work, you know? I even considered calling TMobile and asking them how that would work and if it's really that simple.