The latest version of Skype - version 2.2 - has been released and now includes support for Windows Mobile 6.1. You can download it here .

Even though the call quality is not always flawless, I would still recommend using Skype especially if you get your friends and family to sign up for it and use peer-to-peer calling. I've used Skype and both 5.0 and 6.0 devices and haven't seen a big difference in performance. You do need a 3G...

If you intend to do yourself a favor, use SKYPE and invite your friends to join. Although it is proprietary, at least it works great! You should just have a 400 MHz CPU in your device for voice calls and select a broadband connection (UMTS, HSDPA, WiFi).