4 freefields it's was nice but sometimes not enought.
We add 2 free fields (6 now), and... 2 numerics one (filter with >, <, <>,...; AND/OR) and 2 date based (filter on date, month, year; AND/OR).

The filter system in the Recipient management section let now user filter on those 10 freefields but also now on Subscription date, subscribe/unsubscribe, and HTML/Text subscription.