RBCP has previously used K7, he may be able to comment on the quality.
k7.net is a good service and it's 100% free. You can't beat that. Plus, it seems to give you the caller's caller ID, even when they block their number. I quit using them because I didn't like the low voice quality for PLA Radio.
I'm using Skype now and I'm very happy with them. Great sound quality and the price is fair.