Monitoring VoIP

We are configuring a VoIP system, and need to find some monitoring tools, ie sniffers. Does anyone have any experience capturing and anlyzing VoIP captures and if so what product to you use? How do you like it?
thanks

I've never dealt with VoIP yet. Something I look forward to. If you have Cisco Catalyst 6500's or 7500's (I believe), there is an add-in module called the Network Analysis Module. They have a very easy way of monitoring VoIP traffic over your network. Since the NAM module is added to the chassis, you don't need to use any taps or similar methods. The user who administers the NAM module doesn't even have to have access to the switch. I also know Network Associates has a Sniffer for voice. I'm sure there are many other options out there