"I don't know her name but someone said she was an Emperial Agent so don't cross her..."
Sure we all bash the stock classes, but this one is excellent even compared to custom builds, given you know how to use Illusion. Playing one is a great roleplay for the main and major questlines. I imagine them having a reputation as ilustrated in the hypothetical npc conversation above, sort of like anyone who happened to be rumored as a CIA agent, aka, legal assassin, or an English 007 type might in our world. These are basically legally sanctioned assassins, like the Argonian Agent you might meet in the DB questline. Who better to serve Martin, assassinate Dremora leaders of the Oblvion invasion, and save the day?
The class is excellent for gameplay as well, provided you get access to spell making early on and use your Command spells. I am playing one right now and she has only increased a handful of points on skills other than Illusion, Marksman, and Sneak, which makes her VERY effective for her level. They are very easy to get good attribute bonuses and plus fives every early level except for Agility which you can still increase every level and get a lot of plus threes.
Armorer and Alchemy as minors that you can work up quickly is a common theme of custom characters anyway, which this class has the potential to do as well.
It is imperative however to get custom Command spells, which are so, so good even with a 4 sec duration and a short Inviso effect added. Just a bit ago she was clearing out the bandit cave outside of Skingrad waiting for her meeting with the Count. She cast her Geas III - Command level 20 inviso 3 secs spell, then went invisible which afforded me the opportunity to sit back and drink some beer while three archers fought each other. Even after 30 secs and casting the spell one time, one archer killed the first one then followed the third as it walked back to its place and killed it, then I one-shot the last one with a sneak shot- great fun.
Anyway, just thought I would praise this stock class for no particular reason, except as a suggestion for all those "What is a good stealth class?" posts.