I've been useing Skype a bit. Havn't yet made up my mind about it, but there are a few points I can make.
Compared to MSN or Yahoo the sound quality is clearly better, which is something. Ease of use, functionality and so on I havn't made up my mind about yet (big parts of the program I have yet to touch).
Calls from computer to computer are free, of course, but to call a ordinary phone you have to buy some SkypeOut credit to call with. The rate varies but to make things a bit easier they hav a global rate for the most common places (Argentina (Buenos Aires), Australia, Austria, Belgium, Canada, Canada (mobiles), Chile, Denmark, France, Germany, Ireland, Italy, Mexico (Mexico City, Monterrey), Netherlands, New Zealand, Norway, Portugal, Russia (Moscow, St. Petersburg), Spain, Sweden, United Kingdom, United States (except Alaska and Hawaii), United States (mobiles) and Vatican, but no Finland
), which is 0.017 € or 0.02 $. Other places are as a rule more expensive. As far as I know you can't call Skype from an phone.
As for the security side of things... Well it has during the past year had two (opposed to Firefox's four, with the highes as moderately critical) security advisories, of which secunia rated one Highly critical, but both have been patched, so its 2/2. The security for the communication is encrypted, and I know of no-one who has managed to listen in on a call, but then again I know of no-one who has tried, 0/0. The files you transfered aren't any more secure than with any other instant messenger. Ther have been no viruses for Skype but if it's because nobody has managed to make one or merely for lack of trying I don't know.
If you download only Skype you get no spyware or adware that I've found. And yes, I've been looking.
Considering Skype is planning to conqure the corporate world, they have to think about security. The intresting part, to me at least, is when (if) Skpye shows up on PDAs and smart phones.
Well, is Skype something? Might be. It's not Kazaa, at least, even if the founders are the same.