I've been playing CoH for years and I love that game. But in the past months the game has finally become dull for me. My question is, is Champions fresh and different enough from CoH to grab my interest again? I don't want to buy another CoH.

Yes is the easy answer, it will, as it has done for all us other ex CoH players. However, it may not hold your attention as long as CoH as it can feel similar. It just improves on CoH. For example, the CoH instances got very bland and predicable, CO has some good "open world" missions and some really detailed instances. 
 
So in simple terms, give it a try, and you wont be disapointed. However I dont see myself staying for a long time.

Looks like I'll be giving it a try, and hopefully the content they add will keep me hooked. Now that I think about it, CoH was not the game it is now 4 years ago so I expect Cryptic will do a good job expanding the game.

My personal opinion is to wait. 
Go read the forums and you'll see how much outrage there is amongst the community. At this point in time, Champions Online isn't that happiest place in the world.