SKYPE may well be the most downloaded free software to date.  And it is used by individuals and
corporations globally.  For the average computer user (with sufficient skills) it is almost trouble free
during implementation and in use, well most of the time.  During late Spring 2005 there were some
news of network congestion in the SKYPE network but this has been alleviated to some extent. 
Specialized handsets with features countering VOIP weaknesses are often reported "sold out", a solid
indication of continuing success.  And as it is, a specialized handset with digital compression, echo
management and hidden bias-tone brings out the best in VOIP. 

So, what are the draw-backs?  For the private user, with a modern PC complete with a high-end software
firewall correctly configured, a hardware firewall as well, AV, at least 2-3 ASWs running; no serious problems
at all - the point being, as a PC owner you obviously have no objections to bandwidth use (as you may well
be using 80% for your own conversations) and you have ownership of your own network.  And, referring to
the entire contents of the EULA, everything is OK, so long as you also understand that SKYPE is the most
popular VOIP free-ware and as such may be susceptible to hacking.  But the benefits outweigh any negatives
by far for the private user. Also you may note that even if you specify that SKYPE must not start at boot-up,
every time  you start the program it WILL try to modify your PCs registry settings to do exactly that(!). 
And; after use the program will keep running in background.

Here are the details: