Hmm... not sure if this is worth it unless both parties have monthly plans with xx amount of minutes to use.
Basically, it's like Skype To Go, where you are allocated a local number to call, so you're always paying at a local rate when calling overseas. But with Skype, you still have to pay $/min and a connection fee. Talkster is free!
With Talkster, both you and the person you're calling will be given a local number through text msg. To make the call, dial that number, ask that person to hang up and dial his/her number then you two will be connected through Talkster! Bit complicated eh? Here's a little animation to help you understand how it works. In this case, both parties will be paying for the call, but at their own local rates. So it's essentially free if the minutes is included in both parties' cellphone plans, rather than paying $/min (some local rates can be expensive).
Of course, nothing is absolutely free. Talkster needs to earn its money, and that is through advertising. Prior to connecting the calls, both parties will hear a 10sec local ad. But what is 10sec compared to "unlimited free" talking?? :D