It would be fantastic to be able attach different sets of custom fields to different groups of IP addresses. Different information needs to be documented for say public IP ranges (i.e. URL, ports allowed by ACL, serving Firewall/Router, etc.) than internal IP addresses (i.e. hostname, MAC, switch port, etc.). Right now if a custom field is created the new field is applied to ALL IP addresses in the database. This is less than useful when you need to document all of your address space in one place.
I bought IPAM after using the demo for some basic stuff, evidently I didn't test out the custom field function thoroughly enough. I love this software but I don't think I'll renew the license until I can get rid of all of my spreadsheets.
I hope this feed back helps you improve an already pretty good product!