I definitely can't agree about Hamill being a terrible actor, I do think his acting in the first film was subpar in a few scenes, but it was the great chemistry between Ford, Fisher and Hamill that made it work so well. Personally I think The Empire Strikes Back is proof enough of his acting ability, not every actor can pull off the kind of work he did in that film, I don't think Ford could've done it, he basically have no human to act against in that film, but somehow he managed to pull it off beautifully, a quite underrated performance IMO. I think this is also the case of Lucas vs Kershner as a director.

A little off topic, but is there any truth to the story about Mark Hamill actually falling in love with Carrie Fisher during the filming of Star Wars? I do recall reading that in some interview with him. I can't blame him though.