My not using them has to do with the lack of time I have, not the quality of the product. Pretty much every question you may have is in there. Feel free to read through it. It is a great source of info that I would be in error to think I could match. To give you the skinny on how IDS systems work, we will try to take this from the beginning.
Lets talk about an avg.
It might have a router where they get their internet connection. From there they may have a firewall with three interfaces. One interface going to their DMZ, where they keep all their public servers and what not, one going to the local network, perhaps using some form of NAT, then the third interface going to the router. Normally, you would place the IDS system between the firewall and the router, perhaps on a hub. You can use the IDS system in other ways as well.