i have the sony ericsson tm506 right now and my contract with t-mobile is ending next month. i want a new sony ericsson phone that is an android, but only other phone companies have the android sony ericssons (like at&t) and i dont want to change companies, tmobile is so cheap!

i was thinking of buying an unlocked sony ericcson xperia x10 or the mini on amazon. i was wondering:
1. is it reliable? i know all you have to do is put in the sim card but do unlocked phones tend to have any flaws (such as bad reception or some functions not working) amazon is cheaper but its still kind of expensive for me, i dont want to take any risks
2. what does "international version" mean?
3. some phones have warranties and some dont, is it better to have a warranty for unlocked phones? can i return an unlocked phone on amazon even if it doesnt have a warranty?

i'm still figuring out how this system works so please explain simply as possible, i dont really know any techinical terms. if there is anything else i should be aware about or any other websites with reliable unlocked phones please tell me!

thanks!