No experience using it, but there have been numerous complaints from corporate VOIP users that the VOIP transmissions can provide an entry by which unauthorized users can access their whole (they say "otherwise invulnerable") networks. I wouldn't expect that to be much of a problem for typical individual users, although you might want to investigate before telling someone your credit card numbers over VOIP.

Most of the VOIP systems (that I've seen) for which there have been complaints have used wireless links somewhere in the system, so the vulnerability may be more due to the wireless than just to VOIP.

John