I don't think they suck but I was underwhelmed. The first Iron Man seemed good but I think it's because I had lost faith in Marvel. Thor sucked. Hulk sucked. Captain America sucked. I watched Avengers and was like... really? I waited this long for this? It was serviceable, I guess, but talk about disappointment.
I'm curious why you think this. I could see not liking them, but I don't really see how they could be considered terrible.