As a piece of software I have found it to be very good, but as with any VoIP solution it is only as good as the network (and not just your connection to the Internet, but the entire pathway to your recipient).
They push it as VoIP that "just works" and on that basis it scores highly - I've used it on various devices on various types of network and it has never needed any tweaking. Computer to computer "calls" on a good network are better quality than a regular telephone.
SkypeOut, the service which allows you to terminate one end of the conversation at a regular phone has been a bit more variable. I've used it from Africa and Asia to call the UK and the US at about 1.1pence per minute - I'd not use it for a business call because you do sometimes get cut off, but for keeping up with friends and family it has proven to be very good.