Question about an unlocked sidekick?

I have AT&T and my contract isn't up for six more months. I'm really sick of my phone, it's got a half broken keyboard, scratches everywhere and the cover over the light broke off so whenever I open it up or get a text it GLARES so bright. I was planning on buying an unlocked sidekick from best buy. Now does this mean that once I get it, I can just put my sim card right in and calling and texting will work as normal? I already know you can't use the app store or internet and all that jazz. I'm just nervous that I'm gonna spend 60 dollars and not be able to use the phone.