I've heard many old out of date sites say that it' generates tons of traffic, however, I believe that time has past and now is an out of date article community.
Where are these carnivals being held? If it were that great I'm sure it would be pretty saturated by now. I'll have to give it a try sometime.
Don't assume. Give it a try. I done it before and it can be a great source of traffic for your website if you prepared an excellent content
If your niche is interesting and the community members are supportive, chances are you will be getting the most of it. You can give it a shot and monitor the effect. It worked for me on my other blogs before, although it's quite time consuming and it feels like you're always obliged to create an article for them.