I find it frustrating that you can capture fields when clients sign up but then not be able to sort by any custom fields when sending emails. For instance Gender is an obvious one. You must either create a list OR tag each contact you cant send to only Males.
We have "levels" of clients, and when someone signs up through our website as a Level X Female I am unable to use this information when sending mailshots to Level X Females. I must first Tag this contact manually. CC offered a solution to export the whole contact list then reimport and add custom field as a tag but very clunky.
We have levels as tags and as you found when you import tags via files they add the tags but do not overwrite them. again very frustrating
Also found that if you have a spreadsheet with columns you can import a column as a tag BUT only one column at a time, no way to import multiple tags at once.
Tags in theory sound good but arent that easy to maintain...