I have used them before, and it seems a reasonable service. I didn't get anywhere near the prices quoted due to taxes and whatnot, but it was still decent. A friend of mine has used them for a couple of years now to call Uruguay, and he said that one of their newer cards seems to really have no taxes and gives excellent value. Give them a go anyhow - they send you the access number by email, and neither of us have had any major problem with them.

Like the previous poster said though, if you are looking for a cheap call then they are fine. If you are looking for excellent customer service beyond the basics then look elsewhere.