Well.
The quality is near cd audio quality, depends on what software you use and if you go for the hardware VOIP solution, you actually don't phone IP's as such but you get a SIP number, which is an online number associated with your internet ip address, you can pick up very cheap deals for pay as you go VOIP now, pay as little as £15 and get a USB handset and a SIP number and £5 free top up for your VOIP account.
It's good stuff, it's worth spending the money on, do make sure though, make sure your ADSL line isn't congested, as in you can use VOIP when downloading etc, and also VOIP does use quite a bit of your bandwidth if your broadband is capped.
Chris,