Originally we did use early open source VOIP projects such as OpenH323. OpenH323 was great, but it needed to be replaced as we moved to SIP and required reliability beyond what OpenH323 offered.
Asterisk is in a similar place - it is a great project that has seen some great early success in voip. I have heard that Vonage, for example, uses it in their voicemail system. I also use it at home and we have several projects at work in the research phase that incorporate it.
Asterisk is not reliable enough for our production environment today - reboots every few weeks to few months are common. As a project it is similar to where Linux was 5+ years ago - plenty of momentum but not quite ready for mission critical use. I have no doubt Asterisk will become as pervasive and reliable as Linux and other leading open source projects have though. Asterisk is an extremely flexible, easy to work with project; and the people involved are also easy to work with and know telephony very well.