grimas - I've used BigZoo.com (which also runs/owns OneSuite.com). All I can say is good things about their service.
Although they could always use some improvements, such as:
1) Call quality to obscure countries isn't as good as AT&T. However if you're calling within the USA, Canada, Asia, Europe, Australia, New Zealand, etc. it sounds clear and crisp. Why pay a monthly fee for long distance when you can get super low rates?
2) If you call during peak hours their toll free access line is really busy, that's what your redial button is for, use it! After re-dialing about 10 times you finally get through.
3) Remember if you call from a payphone there's a 55 cents connection fee.
Otherwise this is an AWESOME service and you can test it out for only $5, I usually buy my time in increments of $20.