I was looking for an extension for a complex project (intranet site) with special needs. I tried other CCKs before I found this and give a try.
I have to say with Cobalt there is no limit you can build everything. There are lot of field types and you can modify them to fits your needs. Create own templates for every field and for every content type is another big benefit. The support is excellent too. The only thing that I miss is a complex export/import tool. I hope in next version it will be added.
BUT THERE IS ONE VERY SERIOUS PROBLEM!!!
If you create catalog system with lots of extra fields (40+) it will dramatically slow down your site in category view. Even if you create custom template to show only few extra fields in category view. The reason of this (I think) is the way extra fields are stored in the database. All of the extra fileds for an item is stored in one database field. If you display for example only 5 extrafields from 40 for each item in category view the system will looping through all extra fields because they are stored in one database field. I tried to get help from developers but there is no answer and no solution. The extra fields would give the real power of a CCK but this way it unusable for items with lots of properties. I hope this will be fixed if so this will be the number one extension.