has anyone used available to promise which is part of demand planning module? how useful is this tool? is it easy to use? etc..

Super simple. Set the ATP related days to 90 on the item records on the location subtab. 99% of people use ATP with Look Ahead type on the item record, Discrete doesn't fit many business cases. You'll see a new box/button next to Expected Ship Date on the SO line when you click into that box; you can click to open a pop up to make determinations. There will also be a new transaction with similar functionality. Key is it is a BASIC functionality. It takes existing supply and demand orders. It is not Capacity to Promise.

Thanks, Rich - this is very useful. We're considering Demand Planning with the ATP as a key feature.