Sorry, it's not a poem.
I've built my Daemons list around speed. One of the primary issues with Daemons is a general lack of mobility once they hit the ground. If your opponent deploys well you can often find yourself out of position, especially if they've made you go first and show your hand. Thus, I use the beauties that are Fiends, whose primary value over Bloodcrushers, to me, is their speed. I also enjoy a Bloodthirster and a winged Nurgle Prince.
To help with this, I have incorporated what I feel is one of the most useful units in the Daemons codex in my list: the Masque. WildEyed played a Daemons player at last year's Patriot Games who used her, and he said that it was a very annoying unit to deal with if used properly.
I understand the weaknesses of the Masque, so let's just quickly go over them.
1. She isn't an IC; therefore, she can't join a unit and must scamper around by herself, leaving her incredibly susceptible to a quick death if your opponent focuses fire.
2. She only has T3 and 2 Wounds. This, coupled with #1, can lead to a quick demise if your opponent realizes the potential annoyance she presents.
3. A 3+ Invulnerable is nice, but it's only a 3+.
Now, what about the cool stuff. With the ability to fire the Pavane 3 times per shooting phase, the Masque can help you in a number of ways.
1. The bane of my Daemons list is massed fire, so a single failed charge can be a game changer. The Masque can pull enemies up to 6" closer, which can make a vast difference.
2. We have a general lack of grenades. Fiends are quick, as is the Bloodthirster, but neither has grenades, and assaulting into cover negates that speed completely. The Masque can help pull enemies out of cover, ensuring a Difficult Terrain-free assault.
3. The Masque can also ensure a multi-charge, allowing you to take out multiple units at once and either tying them up or taking them out.
4. If you don't want to fight something, you can frequently move it in such a manner so as to avoid it. I played my buddy's Grey Knights, and I fed his Paladins a unit of Fiends. After my boys died, badly, the Masque simply threw the Paladins back into cover, over and over again for the rest of the game. With the never-ending Difficult Terrain tests, I never had to mess with them again.
5. I love turning Dreadnoughts around so that you can shoot them in the rear armour. It's particularly fun against Ironclads.
6. Lastly, assuming she manages to survive until the last turn, she can pull enemies out of that 3" sweet spot and deny them objectives.
Essentially, my army does not require the Masque to win, but she does help it work more fluidly and efficiently. She's a force multiplier. If you drop her in your second wave, she can make her points back on her first turn by feeding units to your more deadly stuff.
So, that's my "Ode to the Masque." I think she's an under-utilized unit and one that can help you significantly if she matches your play style.