I've used Skype. And I like it. It has a really simple user interface and does what it promises very well. The call quality is generally great, but I've now uninstalled it from all my PCs preferring instead to use the new Windows Live Messenger client. Why? Because Skype, for all its merits, can start behaving as a bandwidth hungry super-node. What does that mean? It means that with Skype installed and running my internet bandwidth was potentially being used by other Skype users without my knowing about it. Which could mean that my PCs consume more electricity and I suffer a slower internet connection. Not exactly the reasons I signed up in the first place.
Ja.net carried out an interesting study recently to assess the impact that installing and running the Skype client can have on bandwidth usage. The findings were eye-opening to say the least. They installed Skype on a PC connected to a 1Gbit/sec internet connection. They didn't use Skype to make or receive any calls and no other applications were used on the PC. They then monitored the PC's internet activity over a 2 week period.
What they discovered is that their Skype client appeared to start behaving as a super-node, an intermediary client acting on behalf of other Skype users. In essence, what happened is that their PC became a mini communication hub for Skype.
If you want to get the best performance out of your Internet connection I recommend you read through the report at http://www.ja.net/development/voip/skype&janet.pdf and decide if Skype is right for you. This is particularly important if you have a capped ADSL connection or pay for your internet usage as you go. With the Skype client running in the background on your PC (the default behaviour after it is installed) you could find it generates significant network traffic by acting as a proxy for other Skype traffic. Of course, you can still continue to use Skype if you wish to make calls using their service. But you may find it preferable to only run the Skype client when you need it instead of leaving it active all the time. The downside of this approach is that you will not be able to receive incoming Skype calls without prior arrangement. The upside is that you won't be loaning your expensive internet connection to people you don't even know.
[Thanks to Mark Deakin for educating me about the way Skype works!]