It depends on the person. I'd much rather people were straight and honest with me, rather than wooly and said the dreaded "That's nice".

Also it depends on experience I guess, if you're just starting to learn something out of your comfort zone, you don't really want harshly honest critique of your (probably lame) efforts. You want mainly encouragement. Once you become more confident though, and believe in your own abilities, you just want the truth.