I just finished my first full campaign, as England, and I made extensive use of Assassins.

A few tips/observations on making use of Assassins:

1. You need a lot of assassins. Any area of my territory or enemy territory that I have my eyes on should have at least one assassin able to strike at any given time. I had at least 15-20 running around.

2. You have to babysit them. This is true for all agents of course, but especially so for Assassins. Make sure you give every assassin a mission whenever possible. My favorite training mission is a rebel general. I had several rebel armies in my territory with 3 or 4 assassins surrounding it.

3. Give them 95% missions whenever possible. Chance to succeed seems to have no effect on chance to gain skill. I've skilled from rank 9 to 10 on a 95% mission.

4. Decide which city is going to be your assassin factory early on, and beeline for assassin's guild. With Assassin's guild HQ and Pleasure Palace, assassins regularly pop out with 6 skill and multiple traits that can go up, putting the assassin only a few missions away from rank 10.

At first I was shocked at how weak and useless Assassins were. Then I decided to really try to build them up and was shocked at how easy it is to have a death squadron of assassins early in the game. It's kinda cool to "clean" the map in a few turns. No annoying merchants, diplomats, princesses, etc. running around.....unless they're mine of course. Muhahah.