I use SkypeOut and SkypeIn on my n900 everyday. My SkypeIn numbers are Australian and US.

When at home, I call using skype out over WiFi (3G when out). It costs a fraction of my regular prepaid rates - I pay ~$10/month for more than enough prepaid 3G data on my n900, and barely touch the broadband usage.

Skype Video in PR1.2 is going to kick ***. When I'm in the US on a trip, I'll be able to call home and 'see' my wife and boy.

Skype was one of the main reasons I bought a n900 - second only to Linux and the goodies that brings (VPN, SSH, root access etc).