diva7,
We built a crawling track for our daughter but she was already almost 6 months old when we started using it. I regreted inmensibly not having built earlier. We didn't use it for sleeping purposes, but we'll definetly use it for sleeping purposes with our next baby. If I'm not mistaken texaslady, one of the members here, did use it for sleeping purposes with her baby. She said her baby was crawling since he was very little and they had to wake up to flip him so he could go the other direction. Based on this she said they would do a crawling track around the bed with the next baby. That is the way is shown in Doman's book. I think she is pregnant right now, so she may be able to give you a better idea of what she is planning to do and give you more information about her experience with it and her baby.
Hope this helps a little bit. Let me know if you have an specific question and hopefully we can find an answer