I had difficulty the first time I tried to buy credit, but tried again later and had no problem.
My experience of the sound quality with SkypeOut has been unacceptable, though. My reception of the person on the other end is fine, but they hear me with varing degrees of echo-from-in-the-barrel or electronic-sounding distortion, with periods of my voice dropping out completely, from 5% to 30% of the time, depending on the call.
The real killer is that I've checked my sound quality with Skype's Echo 123 test service several times, and the quality of both transmitting and receiving is flawless. I'm calling from the East coast to West coast U.S., and I have to conclude that Slype's servers are overloaded at the point where they are connecting back into the normal phone network. I'll check back later, but I can't see using Skype until I can depend on it...