ArmorGames was the first experience I've had with any sponsorship and it was better than what I expected.
They're very reliable, friendly, helpful and ask you first before doing anything with your flash.
ArmorGames helped the exposure and traffic of my games by posting them at other large websites/flash communities, I never knew existed.
(heck my site got slammed with literally hundred thousands of hits in the first two weeks and they keep on coming. I had the Armorgames and my site's link right next to one another.)
The requirements for placing AG links on your flash titles is simple, doesn't clutter your flash, is not distracting, and that animated intro looks great.
If I were asked by a few sites for sponsorship, AG would be somewhere at the very top of the list!
I haven't tried out any of those contests yet, but if I had more time I'd definitely try it out!!